Add a test that requires all known WGSL language features to be supported

[kainino0x]

See test description.

Tested in Chrome Stable and Canary, got expected results.

Issue: https://crbug.com/557328524

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Tests avoid over-parameterization (see case count report).

When landing this PR, be sure to make any necessary issue status updates.

[github-actions]

Results for build job (at b4e98b4):

+webgpu:shader,validation,parse,requires:have_all_language_features:* - 14 cases, 14 subcases (~1/case)
-TOTAL: 285370 cases, 2347158 subcases
+TOTAL: 285384 cases, 2347172 subcases