Passing the current position to a `process` directive
When I declare a `process` directive (specifically a custom `process`), is there any way I can pass the current position to the `process` function?

If not, do you think this is something that makes sense to implement? I had a quick look, and I think it may be possible to define a new `Identifier` in (`io/kaitai/struct/format/Identifier.scala`), e.g. `val POS = "pos"`. Then in your .ksy file, if you had:

```yaml
id: foo
size: 10
process: do_something(pos, ...)
```

The `pos` argument would be translated to the Kaitai Stream's get position function. E.g. in for Python this would translate to:

```py
_process = DoSomething(self._io.pos())
self.foo = _process.decode(10)
```

What do you think? Do you think this makes sense? I want to do this because I want to do some custom processing that depends on the current position of the stream, but I'm not sure if this is too niche and not really a general feature that would actually be useful.