Remove Shader weak_handles from bevy_pbr meshlets (with one exception).

[andriyDev]

Objective

• Related to "Always Strong" Asset Handles / Remove asset weak handles #19024

Solution

• Use the new load_shader_library macro for the shader libraries and embedded_asset/load_embedded_asset for the "shader binaries" in bevy_pbr meshlets.

The last piece that I haven't handled here is MESHLET_VISIBILITY_BUFFER_RESOLVE_SHADER_HANDLE. This is in an awkward situation that prevents us from using the standard solutions I've been using thus far. Essentially, a shader with this import path needs to be present, but we also need to be able to replace it if meshlets really are loaded. Embedded assets don't really play nicely with that, so we will need to figure out a solution there (or maybe wait for WESL to hopefully resolve this situation).

Testing

• meshlet example still works.

P.S. I don't think this needs a migration guide. Technically users could be using the pub weak handles, but there's no actual good use for them, so omitting it seems fine. Alternatively, we could mix this in with the migration guide notes for #19137.

[atlv24]

I would prefer to just merge #19318 and avoid merge conflicts

[andriyDev]

@atlv24 I get not wanting to deal with merge conflicts, but I'd prefer these two things be split. That way we can revert each independently if necessary.

But hey if that PR gets in first, I'm ok to close this one.

[atlv24]

meshlets is experimental, i dont think breakages warrant reverting there, especially this early in the cycle. if something breaks ill fix it, id really prefer not to rebase that work as its not all mine