Mesh shader HLSL writer

[inner-daemons]

Connections
Part of #7197
Closes #7219
Related to #8739
Depends on #8756

Description
Adds support for mesh shaders to HLSL writer. There still exist some things to do:

• Possibly disable some mesh outputs that are unused in the fragment shader

Testing
Existing tests, snapshots

Squash or Rebase?
Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[inner-daemons]

@cwfitzgerald One question here, not probably too important:

Can you explain to me whats going on in naga::back::hlsl::Writer::write_ep_output_struct? It seems to be removing some HLSL outputs but as far as I can tell DX12 allows the vertex shader to output @location things unused by the fragment shader. I'm not sure if I should be doing this in the mesh shader writer.