feat: add an option for preserving file extensions of .ts and .tsx files

<!-- DO NOT IGNORE THE TEMPLATE! Thank you for contributing! Before submitting the PR, please make sure you do the following: - Read the [Contributing Guide](https://github.com/antfu/contribute). - Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate. - Provide a description in this PR that addresses **what** the PR is solving, or reference the issue that it solves (e.g. `fixes #123`). - Ideally, include relevant tests that fail without this PR but pass with it. --> ### Description A quick PR for adding an option to preserve file extensions in generated `auto-imports.d.ts` Close #601 ### Linked Issues #601 ### Additional context <!-- e.g. is there anything you'd like reviewers to focus on? -->