feat: allow apply condition to be a function

[andylizi]

Description

apply can now be a function. Please see #4536 (review) for the original motivation.

function myPlugin() {
  return {
    name: 'client-build-only',
    apply(config, { command }) {
      return command === 'build' && !config.build.ssr
    }
  }
}

Additional context

Unfortunately we won't have ResolvedConfig ready when we check for apply and that's not easy to change (we don't want to call config hook on a plugin if it's ultimately not going to apply, for example). I eventually decided that UserConfig and ConfigEnv should probably be enough for most use cases, and those can always fallback to if (...) return pattern if necessary.

It felt a bit awkward trying to write documentation for this (not a native English speaker), any wording suggestions are welcome :D

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• 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.

[brillout]

Love it 😍

[antfu]

Thanks! Since this is a feature to the config interface, we will leave it to the meeting on Friday and asking Evan for input.

[andylizi]

Done!

By the way, should I update the implementation in #4536 in this PR while we are at it?

[patak-dev]

I think it is better to do another PR to update the implementation later

[patak-dev]

haha, just saw @antfu comment, @andylizi looks like this is your call then 😉

[patak-dev]

Thanks for the PR @andylizi, it would be great to be able to clean up plugin-legacy 👏🏼