feat(vitest): allow overiding package installer with public API

[sheremet-va]

Description

Closes #4864

Proposed API:

• env VITEST_SKIP_INSTALL_CHECKS that disables all install checks - Vitest assumes all dependencies are installed
• VitestPackageInstaller class with ensureInstalled method (exported via vitest/node) - other implementations can override it if needed and provide a new class to vitestOptions when starting Vitest:

await startVitest('test', [], {}, {}, {
  packageInstaller: new CustomPackageInstaller()
})

cc /@Jinjiang

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Tests

• Run the tests with pnpm test:ci.

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Changesets

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[Jinjiang]

@sheremet-va it looks great! May I confirm what would happen if I return false in the ensureInstalled() method?

[sheremet-va]

Depends on the caller. In most situations, it will exit