[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.

[andyleiserson]

One suggestion, but it's not critical and otherwise this looks good. Thanks for the PR!

[andyleiserson]

I'd suggest making a helper Token::is_start_of_compound_statement for matches!(token, Token::Attribute | Token::Paren('{')). It could be used here and on line 2124 where fn statement parses compound statements. This better encapsulates the sensitivity to a rule from elsewhere in the grammar and reduces the risk of bugs if the grammar changes in the future.

[reima]

That's a good idea! Adding the helper to Token didn't feel right to me, so I've added a free function instead.