current vk.xml confuses glad2
Khronos made some changes to the Vulkan specification XML that appear to confuse GLAD:
```
$ glad --api vulkan --out-path="$PWD" c --loader --alias
[17.02.2023 02:56:32][DEBUG     ][glad.plugin   ]: loaded language c: <class 'glad.generator.c.__init__.CGenerator'>
[17.02.2023 02:56:32][DEBUG     ][glad.plugin   ]: loaded language rust: <class 'glad.generator.rust.__init__.RustGenerator'>
[17.02.2023 02:56:32][DEBUG     ][glad.plugin   ]: loaded specification egl: <class 'glad.specification.EGL'>
[17.02.2023 02:56:32][DEBUG     ][glad.plugin   ]: loaded specification gl: <class 'glad.specification.GL'>
[17.02.2023 02:56:32][DEBUG     ][glad.plugin   ]: loaded specification glx: <class 'glad.specification.GLX'>
[17.02.2023 02:56:32][DEBUG     ][glad.plugin   ]: loaded specification vk: <class 'glad.specification.VK'>
[17.02.2023 02:56:32][DEBUG     ][glad.plugin   ]: loaded specification wgl: <class 'glad.specification.WGL'>
[17.02.2023 02:56:32][INFO      ][glad          ]: getting 'vk' specification from remote location
[17.02.2023 02:56:32][INFO      ][glad.opener   ]: opening: 'https://raw.githubusercontent.com/KhronosGroup/Vulkan-Docs/main/xml/vk.xml'
[17.02.2023 02:56:32][INFO      ][glad          ]: generating vulkan:None/vk=None
Traceback (most recent call last):
  File "/home/steven/Development/glad-venv/bin/glad", line 33, in <module>
    sys.exit(load_entry_point('glad2', 'console_scripts', 'glad')())
  File "/home/steven/Development/glad/glad/__main__.py", line 181, in main
    feature_sets = list(select(specification, api, info) for api, info in apis)
  File "/home/steven/Development/glad/glad/__main__.py", line 181, in <genexpr>
    feature_sets = list(select(specification, api, info) for api, info in apis)
  File "/home/steven/Development/glad/glad/__main__.py", line 178, in select
    return generator.select(specification, api, info.version, info.profile, extensions, config, sink=logging_sink)
  File "/home/steven/Development/glad/glad/generator/c/__init__.py", line 321, in select
    return JinjaGenerator.select(self, spec, api, version, profile, extensions, config, sink=sink)
  File "/home/steven/Development/glad/glad/generator/__init__.py", line 60, in select
    return spec.select(api, version, profile, extensions, sink=sink)
  File "/home/steven/Development/glad/glad/parse.py", line 657, in select
    raise ValueError('Invalid API {!r} for specification {!r}'.format(api, self.name))
ValueError: Invalid API 'vulkan' for specification 'vk'
```

Seems to be a result of Khronos adding comma-separated API names, e.g.:
```
<feature api="vulkan,vulkansc" name="VK_VERSION_1_0" number="1.0" comment="Vulkan core API interface definitions">
```

I tried doing a quick workaround:

```diff
diff --git a/glad/parse.py b/glad/parse.py
index a133b40..807f923 100644
--- a/glad/parse.py
+++ b/glad/parse.py
@@ -450,7 +450,8 @@ class Specification(object):
         features = defaultdict(dict)
         for element in self.root.iter('feature'):
             num = Version(*map(int, element.attrib['number'].split('.')))
-            features[element.attrib['api']][num] = Feature.from_element(element)
+            for api in element.attrib['api'].split(','):
+                features[api][num] = Feature.from_element(element)

         for api, api_features in features.items():
             features[api] = OrderedDict(sorted(api_features.items(), key=lambda x: x[0]))
```
but this leads to other issues I haven't dug into yet:
```
$ glad --api vulkan --out-path="$PWD" c --loader --alias
[17.02.2023 03:00:54][DEBUG     ][glad.plugin   ]: loaded language c: <class 'glad.generator.c.__init__.CGenerator'>
[17.02.2023 03:00:54][DEBUG     ][glad.plugin   ]: loaded language rust: <class 'glad.generator.rust.__init__.RustGenerator'>
[17.02.2023 03:00:54][DEBUG     ][glad.plugin   ]: loaded specification egl: <class 'glad.specification.EGL'>
[17.02.2023 03:00:54][DEBUG     ][glad.plugin   ]: loaded specification gl: <class 'glad.specification.GL'>
[17.02.2023 03:00:54][DEBUG     ][glad.plugin   ]: loaded specification glx: <class 'glad.specification.GLX'>
[17.02.2023 03:00:54][DEBUG     ][glad.plugin   ]: loaded specification vk: <class 'glad.specification.VK'>
[17.02.2023 03:00:54][DEBUG     ][glad.plugin   ]: loaded specification wgl: <class 'glad.specification.WGL'>
[17.02.2023 03:00:54][INFO      ][glad          ]: getting 'vk' specification from remote location
[17.02.2023 03:00:54][INFO      ][glad.opener   ]: opening: 'https://raw.githubusercontent.com/KhronosGroup/Vulkan-Docs/main/xml/vk.xml'
[17.02.2023 03:00:54][INFO      ][glad          ]: generating vulkan:None/vk=None
[17.02.2023 03:00:54][INFO      ][glad          ]: no explicit version given for api vulkan, using Version(major=1, minor=3)
[17.02.2023 03:00:54][INFO      ][glad          ]: adding all extensions for api vulkan to result
[17.02.2023 03:00:54][DEBUG     ][glad.parse    ]: type VkQueryPoolCreateFlagBits with category enum but without <enums>
[17.02.2023 03:00:55][DEBUG     ][glad.parse    ]: type VkDeviceCreateFlagBits with category enum but without <enums>
Traceback (most recent call last):
  File "/home/steven/Development/glad-venv/bin/glad", line 33, in <module>
    sys.exit(load_entry_point('glad2', 'console_scripts', 'glad')())
  File "/home/steven/Development/glad/glad/__main__.py", line 181, in main
    feature_sets = list(select(specification, api, info) for api, info in apis)
  File "/home/steven/Development/glad/glad/__main__.py", line 181, in <genexpr>
    feature_sets = list(select(specification, api, info) for api, info in apis)
  File "/home/steven/Development/glad/glad/__main__.py", line 178, in select
    return generator.select(specification, api, info.version, info.profile, extensions, config, sink=logging_sink)
  File "/home/steven/Development/glad/glad/generator/c/__init__.py", line 321, in select
    return JinjaGenerator.select(self, spec, api, version, profile, extensions, config, sink=sink)
  File "/home/steven/Development/glad/glad/generator/__init__.py", line 60, in select
    return spec.select(api, version, profile, extensions, sink=sink)
  File "/home/steven/Development/glad/glad/parse.py", line 715, in select
    result = result.union(found)
  File "/home/steven/Development/glad/glad/parse.py", line 523, in find
    self._combined.update(self.types)
  File "/home/steven/Development/glad/glad/util.py", line 165, in memoized
    cache[key] = func(*args, **kwargs)
  File "/home/steven/Development/glad/glad/parse.py", line 350, in types
    raise ValueError('extension enum {} required multiple times '
ValueError: extension enum VK_ERROR_UNKNOWN required multiple times with different values
```