refactor(migration): extract import rewriting into separate module

Move import rewriting logic from vite_config.rs to import_rewriter.rs
for better separation of concerns:

- vite_config.rs: Now focused only on JSON config merging
- import_rewriter.rs: Handles all import rewriting (vite/vitest → vite-plus)

This makes the codebase more maintainable and the module names more
accurately reflect their responsibilities.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: fengmk2