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

[Jonas Finnemann Jensen]

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: #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.

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

[Lukas Klingsbo]

Super, I'll add it once I'm back in Sweden!
(I'll do the same for Flame too)