WGSL: Add a skeleton for enable-extensions/enable …; that reports nice errors

[ErichDonGubler]

Connections

• Based on Add a skeleton for directives that reports nice errors #6352.
• Paves the way for feat: implement F16 support in shaders #5701, support enable clip_distances extension #6236, and Converge dual-source blending with the WebGPU standard #6402.

Description

This PR is much like #6352, but for the enable directive; it unblocks work on enable extensions in WGSL by serving as a foundation on which to build individual extension functionality. It does the following:

1. Build on Add a skeleton for directives that reports nice errors #6352 by parsing an enable extension list after we encounter an enable in the directive parse path keyword, instead of emitting an unimplemented directive error.
2. Create an enum modeling all known extensions, including standard-but-unimplemented ones.
3. Add nice diagnostics for known and unknown extensions. If a known, but unimplemented, extension is requested, provide a hint that points users towards the appropriate Naga issue, so they can let us know they were blocked.

Note that this PR does not actually implement any extensions yet—see also the UnimplementedEnableExtension::tracking_issue_num method added in this PR for references to follow-up work for those.

Testing

• This functionality is exercised in feat: implement F16 support in shaders #5701.
• More exhaustive tests will be added in WGSL: Add tests for enable directive parsing #6444.

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.

[ErichDonGubler]

@teoxoy: This PR is exercised by #5701; any objection to punting tests to a separate PR, so we can keep yet another blocker for it resolved?

[teoxoy]

That's fine with me.

[teoxoy]

LGTM