Add use_repo_add and use_repo_remove commands (#1152)

* Preserve compactness of `use_repo` and `use_extension` calls

* Add `use_repo_add` and `use_repo_remove` commands

These commands can be used to programmatically update `use_repo` calls
in MODULE.bazel files. In the future, Bazel will emit buildozer fixups
relying on this command when it detects that such calls are out-of-date.

* Adapt label normalization to be closer to upstream

* Fix CI