WESL: Suport importing shader from asset source

[beicause]

Objective

Fixes #20285

Solution

If wesl shaders import a dependency from a named asset source (e.g. embeded), we register the dependency into the bevy_asset module so it can be imported as import bevy_asset::{asset_source}::{asset_path} (my current proposal, open for discussion). super also works.

Testing

Modified the shader_material_wesl example to import embeded wesl for testing.

[beicause]

It's bad to manually import every dependency. But Shader::from_wesl requires parsing dependencies in advance when loading (such as naga_oil::compose::get_preprocessor_data), which wesl-rs currently doesn't provide.

[tychedelia]

Hey @beicause, I'm currently working on porting all our shaders to WESL and will need to account for some of this. I'm not sure we can merge this as-is or if that work will conflict there, but I may cherry-pick some of this there. We definitely need to have a bigger conversation about import paths as part of this. I'm not sure I love the bevy_asset::embedded pattern, however, because embedded assets aren't really "in" bevy_asset.

[beicause]

In my opinion, it is necessary to register the asset source in a specific package (not necessarily using the approach from this PR).

If it is registered at the root module, such as import package::{asset_source}, it may occupy a actual folder.

For example https://docs.rs/wesl/latest/wesl/struct.StandardResolver.html#method.add_constant also registers constants in a non-existent "constants" package.