feat(wgsl): recognize enable primitive-index;

[ErichDonGubler]

Connections

• Contributes to, but does not resolve, Support enable primitive-index; in WGSL #8236.

Testing

• TODO: Figure out what we should test here.

Squash or Rebase?

squashplz

Checklist

• If this contains user-facing changes, add a CHANGELOG.md entry.

[ErichDonGubler]

Not sure if we needs tests or a CHANGELOG yet. Thoughts, @gfx-rs/wgpu?

[andyleiserson]

I created some infrastructure for testing extension availability, but it's for the case where the extension is supported.

I do think it's worth adding a test, mostly to document the state of affairs. Nothing fancy, just a wgsl_errors test for the error that gets raised when enable primitive-index appears in WGSL.