Better compatibility for WGSL on WebGL2

Since WGSL does work when targetting WebGL2, it would be lovely if we could add a form of preprocessing for cases where discard to exist in or get imported to a vertex shader source.

That way we can just reuse the WGSL library for WebGL2 targets instead of needing specialized/seperate shaders.

Currently importing bevy_pbr::pbr_functions in a vertex shader source will fail at the validation step in all major browsers.
https://github.com/bevyengine/bevy/blob/main/crates/bevy_pbr/src/render/pbr_functions.wgsl#L24

It's fairly difficult to debug, since the only proper hint we get is the following:
`Internal error in VERTEX shader: ERROR: [line-here]:[column-here]: 'discard' : discard supported in fragment shaders only
`

Or maybe it should just be upstreamed to wgpu/naga.

Start with crates/bevy_pbr/src/render/pbr_functions.wgsl around line 24 and reproduce the validation failure when importing bevy_pbr::pbr_functions into a vertex shader targeting WebGL2. Trace the relevant preprocessing or validation path, and determine whether the compatibility change belongs in Bevy or should be upstreamed to wgpu/naga; done means the WGSL library can be reused for WebGL2 without the vertex-shader discard error.

Written by the indexing model from the issue text.