chore: introduce vitests for tests

[danez]

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Summary

This introduces vitests for test files with mjs extension.

For integration tests the CI uses sharding again to split up tests into multiple workers, right now with one test this is kind of useless, but will be important later. In PRs the flag --changed is used to only run tests that have changes.

vitest is clever enough to rerun all tests if package.json and shrinkwrap change. :)

The Jobs have also been renamed and are now called

• Tests / E2E
• Tests / Unit
• Tests / Integration
• Tests / Legacy Integration (which will be going away)

I was wondering what the all job is doing and found this: #4164 (comment)
So it was meant to be the only job required from this workflow, which isn't the case right now as all test jobs are required. I opted to remove the all job for that reason, but happy to restore.

Ref #5265

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[github-actions]

📊 Benchmark results

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Legend

• T (current commit): 247 MB

[danez]

All unused

[jackiewmacharia]

I've added a few nitpick suggestions, all in all, this looks good.