wgsl: Replace CaseList with Case[]

[zoddicus]

Fixes #3458

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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 in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

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

[zoddicus]

PTAL, my brain always goes CaseList should be Case[] when I am looking at code that uses it, so I made a PR that changes it.

I think this is more idiomatic, but don't actually feel super strongly about it being 'correct'. If you prefer having the CaseList type, I will just abandon this and hopefully stop thinking I should change CaseList to Case[].