Add CTAGS generators / module

[sellout]

Lots of tools generate CTAGS, but it’s important that they stay up to date and the relevant tools can be configured in project-manager without requiring that tags files are committed to the repo.

E.g., for Nix, nix run github:lf-/nix-doc -- tags will build them. If configured, Project Manager should both build them and provide a check to ensure they’re current (if they’re committed).