Re-include `isRepoSearch` in `isRepo`

[fregante]

As described in refined-github/refined-github#5739 (comment)

1. Drop isRepoSearch exclusion from isRepo
2. Add hasRepoHeader (isRepo && !isRepoSearch)
3. Ensure that other detections aren't affected by this change (e.g. isSingleFile depends on isRepo, but it never matched isRepoSearch in the first place, so that's fine)
4. Release breaking version
5. All isRepo usage in the extension to hasRepoHeader