feat(build): add copyPublicDir option

[bluwy]

Description

This PR adds build.copyPublicDir: boolean option to control whether to copy the publicDir to outDir. Default true.

Additional context

Case

Vite default build copies files in publicDir to outDir. In common cases, this isn't useful for SSR builds as the client build already contains the publicDir files. To avoid unnecessary files, SvelteKit and Astro sets publicDir: false to turn it off.

Problem

However, turning it off disallows import img from '/favicon.png', which is possible with plugin-vue's asset transform:

<img src="/favicon.png" alt="" />

Which would error on SSR build as Vite can resolve /... imports from the publicDir. (withastro/astro#5063) (Extra base/publicDir discussion: #9276)

Solution

1. New option like this PR
2. Don't copy publicDir in SSR builds by default (could be breaking)
3. Disable imports in publicDir as documented (Though the discussion above say otherwise Allow importing public files for base support #9276)

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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 Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• 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.

[sapphi-red]

I think this will fix #10082 (comment), too.
My current opinion is to go with this PR for Vite 3 and later go with 3. But I didn't take time to consider about this deeper so I might change my mind.

[bluwy]

I think no3 is a bit of a special case that probably needs discussion too, but yeah with Vite 4 close, we could consider a nicer overall solution for this too.

[patak-dev]

We are looking to stabilize this feature. Request for feedback here:

• Stabilizing `build.copyPublicDir` #13807