fix: apply LiteralUnion to more types for better type safety

[chenjiahan]

Summary

Updates some type definitions to use the LiteralUnion type for better type safety.

Checklist

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

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[Copilot]

Pull Request Overview

This PR enhances type safety by applying the LiteralUnion utility type to several type definitions that previously used plain string types. This allows for better IDE autocomplete suggestions while still accepting any string value.

Key changes:

• Updated AppIconItem.purpose to use LiteralUnion with specific icon purpose values
• Updated CSSLoaderOptions.modules to use LiteralUnion with CSS module types
• Updated StyleLoaderOptions.insert to use LiteralUnion with DOM insertion points

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/core/src/types/config.ts	Updated AppIconItem.purpose type to use LiteralUnion with standard icon purposes
packages/core/src/types/thirdParty.ts	Added LiteralUnion import and applied it to CSSLoaderOptions.modules and StyleLoaderOptions.insert
website/docs/en/config/html/app-icon.mdx	Updated documentation to reflect the new type signature for purpose property
website/docs/zh/config/html/app-icon.mdx	Updated Chinese documentation to reflect the new type signature for purpose property

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