feat: support obtaining unocss context externally

[hairyf]

Clear and concise description of the problem

Hi, I would like to develop a plugin based on unocss externally (using unocss ctx).

https://github.com/unocss/unocss/blob/main/packages/vite/src/index.ts#L29

However, currently there is no way to access unocss ctx from outside the plugin. Is there any support for accessing it?

Suggested solution

I'm not sure about this, but perhaps you can add a new hook in the plugin? For example:

// use promise or hooks
const deferred = createDeferred<UnocssCtx>()

const plugins = [
  unocss({
    // resolve unocss config and ctgx
    configResolved: deferred.resolve
  }),
  myPlugin(deferred)
]

or use context callback

import { createContextLoader } from '@unocss/core'

function myPlugin() {
  let context
  const loader = createContextLoader()
  loader.load().then(v => context = v)
  return { /* ... */}
}

const plugins = [
   {
      myPlugin()
   }
]

Alternative

No response

Additional context

No response

Validations

• Read the Contributing Guidelines.
• Read the README.md of using the package.
• Already used the Interactive Docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[stale]

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