Auto merge of #19028 - MortimerGoro:webgl2, r=emilio

Kick off WebGL 2.0 implementation

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This PR kicks off the WebGL 2.0 implementation:

- Include WebGL2RenderingContext.webidl and comment unimplemented methods
- Create WebGL2 struct hierarchy with WebGL 1.0 backwards compatibility
- Add WebGL 2.0 entry points to canvas
- Select the correct GL Version on GLContext backends (related PR https://github.com/emilio/rust-offscreen-rendering-context/pull/108)
- Add WebGL version selection in shader compilations
- Create a WebGL 2.0 preference

I tried a complex three.js demo using canvas.getContext("webgl2") and the backwards compatibility worked great.

Next steps:

- I'll add WebGLVersion selection/filtering to WebGLExtensions and move some extensions to core in WebGL 2.0 (e.g. VAOs)
- I'll add the WebGL 2.0 conformance WPT
- I'll create a mega-issue with a the list of all TODO methods for a complete WebGL 2.0 implementation (as @emilio did with WebGL 1.0), so we can start start getting community involvement.

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- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because I will add the entire webgl 2.0 WPT in a different PR

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