test: Migrate to Vitest

[timfish]

This PR:

• Migrates all tests to Vitest apart from the integration tests.
  • This allows us to retain testing on older Node versions and delay introducing breaking changes for now.
• I had to change the CI cache keys because changing a package.json resulted in no lock change and a build failure
• Test usages of the Vite plugin had to add as unknown as vite.Plugin because the Vite versions varied between the plugin and integration tests. This does not impact end users because it will use their Vite version.

[github-actions]

Semver Impact of This PR

⚪ None (no version bump detected)

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Bug Fixes 🐛

• (webpack) Deduplicate webpack deploys by chargome in #875

Internal Changes 🔧

• Migrate to oxfmt by timfish in #880
• Build with Rolldown by timfish in #872
• Remove unplugin by timfish in #876
• Rollup/Vite no longer uses unplugin by timfish in #858
• Esbuild no longer uses unplugin by timfish in #871
• Webpack no longer uses unplugin by timfish in #870

Other

• test: Migrate to Vitest by timfish in #882

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