Add GLSL hover support

[staff0rd]

Eventually I'd like the code completion feature suggested in #17, but for now, this PR:

• Generates GLSL documentation by parsing https://github.com/KhronosGroup/OpenGL-Refpages (see scripts/create-glsl-reference.js)
• Implements GLSLHoverProvider for functions, including parameter and description information
• Implements a link loader/parser for displaying GLSL reference links on https://registry.khronos.org/OpenGL-Refpages/es3/

I'm new to shaders and vscode extensions, so happy to take on any suggestions. I would open further PRs in the future implementing the other providers for GLSL if there is desire to include this.

Screenshot

[helviett]

Seems like repository is dead, so you'd better fork, many will follow 😄