Add Pipeline Overrides for workgroup_size

[kentslaney]

Connections
resolves #4450

Description
The WebGPU spec allows for pipeline overrides of workgroup_size. This is achieved by adding a workgroup_size_overrides field to crate::EntryPoint and registering any override expressions in workgroup_size as an override with the name __workgroup_size_{[012]} since identifiers aren't allowed to start with two underscores in a row. From there, they are resolved to constants with the other overrides and any Some fields in workgroup_size_overrides replace the corresponding workgroup_size.

Testing
This change adds an integration test via wgpu_test, which checks that the workgroup size equals the initialization value by default and that it changes with pipeline overrides.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[cwfitzgerald]

Some nits on the integration tests, looks good on the wgpu side.

[cwfitzgerald]

Tests look good :) Need a naga review still

[cwfitzgerald]

Don't forget to re-request review when things are addressed!

[teoxoy]

Looks great, thanks!