fix(types): add LiteralUnion type and apply to assetPrefix

[chenjiahan]

Summary

• Add LiteralUnion utility type to handle string literal unions with fallback types.
• Use it for assetPrefix to improve type safety.

Related Links

• Literal String Union Autocomplete microsoft/TypeScript#29729

Checklist

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

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[Copilot]

Pull Request Overview

This PR introduces a LiteralUnion utility type to improve TypeScript autocomplete for string literal unions while still accepting any string. The type is applied to the assetPrefix configuration property to provide better developer experience when using the 'auto' literal while maintaining type flexibility.

Key changes:

• Added LiteralUnion<T extends U, U> utility type to enable autocomplete for specific string literals without restricting the type to only those literals
• Updated OutputConfig.assetPrefix to use LiteralUnion<'auto', string> instead of string | 'auto'
• Updated DevConfig.assetPrefix to use LiteralUnion<'auto', string> | boolean for consistency

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/core/src/types/utils.ts	Adds the LiteralUnion utility type with reference to TypeScript issue
packages/core/src/types/config.ts	Imports LiteralUnion and applies it to both OutputConfig.assetPrefix and DevConfig.assetPrefix properties

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[chenjiahan]

Thanks for the review!