Slimmed down WEBGL_debug with just object labels and debug groups

The WEBGL_debug extension exists, however it is not supported among any implementors right now. I have heard through conversation that this is because the debug message infrastructure is relatively painful to support.

In my experience with debugging a lot of WebGL projects, the message infrastructure is not important for my use case.

Maybe it makes sense to spec a version of WEBGL_debug with only these endpoints:

* `pushDebugGroupKHR(DOMString name)`
* `popDebugGroup()`
* `objectLabelKHR(WebGLObject? object, DOMString label)`
* Optional: `getObjectLabelKHR(WebGLObject? object)` (this is easy enough to stash on the application side)

What I would really want is for these debug groups and labels to be passed through to the underlying API. Currently, the most effective technique for debugging WebGL is to use a native API debugger (e.g. RenderDoc), and try to correlate the GL commands to the frame in some other way.

Also: this also isn't in the current version of WEBGL_debug, but another big bonus would be some ability to turn on "debug" shader support, e.g. running fxc.exe with `/Od /Zi`. This will put the shader text and debuginfo inside the generated DXBC, which helps a lot when doing detailed shader debugging.

Start by reading the current WEBGL_debug extension specification and compare it with the proposed pushDebugGroupKHR, popDebugGroup, objectLabelKHR, and optional getObjectLabelKHR endpoints. Review whether implementors can pass labels and groups through to native debugging APIs; the work is done when the reduced extension scope and any shader-debug support are specified and accepted.

Written by the indexing model from the issue text.