fix(types): make PostCSSPlugin type loose

[chenjiahan]

Summary

Use a loose PostCSS plugin type that accepts plugins from different PostCSS versions. This helps avoid type conflicts when user projects use different PostCSS versions.

Related Links

resolve #5439

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR updates the PostCSS plugin type in order to support plugins from different PostCSS versions by introducing a loose type.

• Added a new loose type (LoosePostCSSPlugin) in thirdParty.ts.
• Updated PostCSSOptions and configuration types in config.ts to use the loose type.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/core/src/types/thirdParty.ts	Added LoosePostCSSPlugin and updated PostCSSOptions.
packages/core/src/types/config.ts	Updated import and function signature to use LoosePostCSSPlugin.

Comments suppressed due to low confidence (1)

packages/core/src/types/thirdParty.ts:25

• [nitpick] Using 'any' for LoosePostCSSPlugin achieves the goal of flexibility, but consider using 'unknown' instead of 'any' to maintain a minimal level of type safety while still accommodating multiple PostCSS versions.

export type LoosePostCSSPlugin = any;