Add hasFeature function and disallow features.has

[greggman]

The problem with features.has is that it takes a string isntead of a GPUFeatureName. To prevent typos when checking for features, add a lint warning for features.has(feature) and suggest using hasFeature(features, feature). hasFeature takes a GPUFeatureName and so will warn if the string pass is not a valid feature.

With this change this issue would have been caught before submission.

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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.

[kainino0x]

Nice lint rule, LGTM!