Add GLSL features to Stitch for VSCode

[adam-coster]

The shader-editing experience in the GameMaker IDE is... MEDIUM.

GameMaker recommends using GLSL ES for shaders, since that has the broadest support (particularly on mobile). Unfortunately, GLSL does not seem to have a lot of options for decently-featured editors.

The best VSCode extension I could find is one for WebGL, but it of course doesn't know about the GameMaker built-ins.

Given the relative simplicity of the GLSL language, and the fact that GLSL shaders can only consist of two files (the vertex and fragment shaders), it wouldn't be too large of a project to write a GLSL ES language server and use that in Stitch for VSCode.

Notes and refs:

• GameMaker uses GLSL ES 1.0
• Decent docs are available
• The WebGL extension already has a grammar and misc. other work completed.