fix: narrow defineConfig return type

[fi3ework]

Description

Since defineConfig return an as-is input value and type. We could leverage TS overloading to narrow the returned type as the input type. It will be very useful especially deconstructing a return value from defineConfig and reusing it in another defineConfig.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

After further thought, I think this is good instead of a generic so that the allowed types are stricter.