Support of CMake
**CMake**

For modern C/C++ projects that have more than 1 source/header file, most of them are organized with CMake. i.e. provide a CMakeLists.txt, and may optionally call some xxx.cmake script files. It will be more convenient if spacevim provide cmake (or say, provide better) support.

CMakeLists.txt and xxx.cmake are both using "the CMake language".

**Which platform does the language support?**

- [x] Linux
- [x] MacOS
- [x] Windows
- [x] And more than the aforementioned 3 platforms: Android NDK, iOS, arm linux, qnx, ... if you have an C/C++ compiler, then you have CMake support.

**Any suggest vim plugins for this language?**

There is already syntax highlight for an opened CMakeLists.txt. That's fine, but not enough.

Just like when we have setup the `lang#c`, we type `std::cout` then there's hint for `<<` or some other completion hints, we also need "hints" for CMake:
- the builtin commands completion (think of them as "printf" for C)
- the builtin variables completion (think of them as "INT_MAX" macros for C)
- the variables we have just typed in previous lines

**Additional context**
Just like vscode's cmake plugin (author: twxs)
<img width="592" alt="截屏2022-05-21 23 02 08" src="https://user-images.githubusercontent.com/3831847/169657373-3afbd66a-15ff-4557-a104-ef09c5c76cbb.png">

