fix: correct extension alias order to align with Node.js

[chenjiahan]

Summary

When importing ./foo.js in a TypeScript file, if there are both foo.ts and foo.js, Node.js and TypeScript have different resolution orders:

• TypeScript will resolve foo.ts first.
• Node.js will resolve foo.js first.

Rsbuild should match the behavior of Node.js, because:

1. It maintains consistency with Node.js runtime behavior, avoiding potential surprises when the code runs in production
2. It follows the explicit intent when developers specifically write .js in their import statements

This PR fixes this issue from Discord:

Checklist

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

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