feat: support async plugins

[bluwy]

Description

Based on discussion: #8509

Support config.plugins: (Plugin | Plugin[] | Promise<Plugin | Plugin[]>)[]

Additional context

Turns out deep flattening an array of promises only take 3 lines 😄

Side note:

@poyoho it looks like config.worker.plugins isn't flatten and being handled with falsy, apply prop etc like:

vite/packages/vite/src/node/config.ts

Lines 333 to 343 in c5c424a

	const rawUserPlugins = (config.plugins || []).flat(Infinity).filter((p) => {
	if (!p) {
	return false
	} else if (!p.apply) {
	return true
	} else if (typeof p.apply === 'function') {
	return p.apply({ ...config, mode }, configEnv)
	} else {
	return p.apply === command
	}
	}) as Plugin[]

Is it intentional?

---

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.
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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.

[bluwy]

Marked as draft. There’s one more case in asyncFlatten that I’ll fix tomorrow.

[poyoho]

I think config.worker.plugins needs to flatten and handled apply prop too. 😁

[patak-dev]

We talked about the PR in today's meeting and we decided to move forward. @bluwy once you resolve the conflicts you can merge it 👍🏼

[antfu]

Love it! 🔥