Add support for Godot shaders

[TritiatedWater]

It would be nice if this plugin could support godot shader.
Godot shader is supposed to be similar to glsl which in turn is similar to C.
I can just set VS Code to associate .shader as a glsl shader, but the code assist won't work.

[AlfishSoftware]

You may be interested in the grammar I coded:
https://github.com/AlfishSoftware/godot-files-vscode/blob/develop/lang.gdshader/gdshader.tmLanguage.json
I also added the language variables as snippets, with descriptions from Godot 3 docs:
https://github.com/AlfishSoftware/godot-files-vscode/blob/develop/lang.gdshader/gdshader.code-snippets
Not ideal, but works as a starting point.

[atirut-w]

There's also a new PR for gdshader syntax highlighting: #360

[Calinou]

Implemented by #360.

Note that autocompletion is currently outside the scope of the Godot editor LSP.