Consider adding `topics` to `pubspec.yaml` for better discoverability

[jonasfj]

I'd suggest adding the following lines to pubspec.yaml:

topics:
  - tool
  - repository-management

This is inspired by mono_repo which is a similar tool, though probably less polished.

Topics helps users on pub.dev discover packages, and related packages :D

Originally, filed a PR: https://github.com/invertase/melos/pull/712, but I can't sign the CLA without a CLA-review, so it's probably easier for someone else to decide what topics melos should have and add them to pubspec.yaml.

---

See also: https://dart.dev/tools/pub/pubspec#topics