[naga wgsl-in] Allow trailing comma in switch cases

[reima]

Connections
Fixes #8126.

Description
This change allows a trailing comma in the list of case values inside a switch.

It accomplishes this by checking if a comma is followed by a : (indicating the list of values ends here), or an attribute or { token (indicating that a compound statement follows).

Testing
I've extended naga\tests\in\wgsl\control-flow.wgsl to contain a switch with trailing commas.

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.