Support Vitest 4 in @cloudflare/vitest-pool-workers

[penalosa]

Fixes #11064
Fixes #8369 (defineWorkersConfig is removed)
Fixes #7795 (everything is now encapsulated in a plugin, and so this type of configuration problem is impossible)
Fixes #7339 (we no longer recommend importing env from cloudflare:test. Instead, import env from cloudflare:workers and use withEnv to override the value: https://developers.cloudflare.com/workers/runtime-apis/bindings/#overriding-env-values)
Fixes #10260 (tests added to verify the mentioned libraries can be imported)

This a major breaking change to the @cloudflare/vitest-pool-workers integration in order to support Vitest v4. Along with supporting Vitest v4 (and dropping support for Vitest v2 and v3), we've made a number of changes that may require changes to your tests. Our aim has been to improve stability & the foundations of @cloudflare/vitest-pool-workers as we move towards a v1 release of the package.

We've made a codemod to make the migration easier: wrangler codemod vitest-pool-v3-to-v4, which will make the required changes to your config file.

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• Tests
  • Tests included/updated
  • Tests not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because:
• Wrangler V3 Backport
  • Wrangler PR:
  • Not necessary because:

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