🐛 Bug: Migration mode should not clobber `.gitignore`

[danvk]

Bug Report Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Expected

For an existing project, the .gitignore file has probably accumulated lots of stuff that should be ignored.

I expected the migration script to add to .gitignore or adjust it, not to completely clobber it.

Actual

Running npx create-typescript-app completely clobbers the .gitignore file, replacing it with a two-liner.

Additional Info

I wound up with lots of previously-untracked files added to my repo after running the migrate script over at danvk/literate-ts#246

For example: I was ignoring the coverage directory, which contained HTML reports of code coverage that I would periodically generate. After migration, git wanted to track this.

[JoshuaKGoldberg]

Agreed, makes sense. I think there should be a similar approach to how the migration handles existing dependencies: known no-longer-necessary ones should be removed, and other existing ones retained.