feat(pluginContainer): implement watchChange hook

[developit]

Description

This PR implements the watchChange plugin hook.

Additional context

Currently, observing file changes can only be done via server.watcher.on("change", ...), or somewhat lossily via the handleHotUpdate() plugin hook (operates on Modules rather than Files though, and doesn't run for deletions).

Since the Plugin TS definition and container implementation indicate watchChange() hook is available (it doesn't throw like emitFile), I figured it would make sense to add support for it.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
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• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

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[bluwy]

Makes sense to me. Looks like there's some linting issues that should be fixed by running pnpm format at root.

[bluwy]

We talked about this in the meeting today and are fine merging it 👍 It would be interesting to know if you have any usecase for using watchChange in dev too, so we'll have some guide if we have to change how watchChange works in the future.

[patak-dev]

For future reference, this PR may break plugins that assume that watchChange is only being called during build. But I think this should be rare. It would be good merging it as part of the major though.

[developit]

Thanks for taking this on @patak-dev + @bluwy! I'm heads down landing our Vite switchover so I wasn't able to tweak the tests/formatting.