Generate Web Worker script for every test file

[kainino0x]

This will be needed to launch Service Workers, which cannot be generated at runtime and cannot use dynamic import().

WIP. @beaufortfrancois

Issue: #3345

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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.)

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]

It may or may not replace test_worker-worker.js. It would be nice to avoid the duplicated functionality. But that one is a lot more flexible - it just creates one worker and runs tests on it, instead of having to manage one worker for every test file. But since we need to do that for Service Workers anyway, it's probably simplest to use the same setup and logic for all types of workers.

[kainino0x]

All the code above this point is the same as the old gen_listings.ts. Git didn't detect it as a rename because I added too much code.

[kainino0x]

This landed in #3419