Move Features::PRIMITIVE_INDEX into the WebGPU group

[andyleiserson]

Renames Features::SHADER_PRIMITIVE_INDEX to Features::PRIMITIVE_INDEX (for consistency with the spec, which uses shader_f16 but not shader_primitive_index), and moves it to the WebGPU feature set.

Putting it in the WebGPU feature set results in it being exposed by Deno and available in CTS runs, so, enable the CTS tests for primitive_index. I've also added a cts_runner integration test that checks the feature is exposed, because the CTS silently skips the relevant tests if it is not.

I've also reworded the changelog entry related to enable primitive_index, but the actual functional change was in #8879.

CC @inner-daemons

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.

[inner-daemons]

Yeah LGTM for the most part.

[inner-daemons]

Free parking down the road 🗣

[inner-daemons]

FWIW if we are explicitly saying that not all devices on the API will support it, Vulkan should probably also be (some) since it needs geometry shader support. HLSL is fine because this was added in SM4 (DX10).

[andyleiserson]

I've marked this as draft because the CTS failures in CI appear real, and I don't have time to debug them right now.

#9105 is a failure related to primitive_index on DX12
#9106 is a failure elsewhere in the fragment_builtins suite on DX12 and Vulkan, doesn't necessarily need to be fixed if we enable only the primitive index tests