feat(unocss): support css transformers

[kwaa]

Description

Support for using Transformer.

Tested:

• Variant Group
• Directives

These transformers export wrong types, I'll try to fix them later by creating PR to upstream.

https://arethetypeswrong.github.io/?p=%40unocss%2Ftransformer-variant-group
https://arethetypeswrong.github.io/?p=%40unocss%2Ftransformer-directives

Related Issues

Check List

• Have you read the
  CODE OF CONDUCT
• Have you read the document
  CONTRIBUTING
  • One pull request per feature. If you want to do more than one thing,
    send multiple pull request.
  • Write tests.
  • Run deno fmt to fix the code format before commit.
  • Document any change in the CHANGELOG.md.

[kwaa]

The magic-string has been moved to deps and transformers are enabled by default.

[oscarotero]

Thank you!