Motivation for this change

Similar as the plugins for vim, but now for tmux.

How to use:
They can be loaded by installing the pkg, as well as adding a line in the tmux config like this:

run-shell ${tmuxPlugins.sessionist.rtp}

or, if you have a set of plugins (e.g.: plugins = with tmuxPlugins; [ sessionist yank ];):

${lib.concatStrings (map (x: "run-shell ${x.rtp}\n") plugins)}

I plan on adding the latter in the tmux modules for both nixos and nix-darwin once this gets merged in follow-up PRs.

ps: I didn't add myself to the contributor/maintainer list yet. I'm not sure when that becomes needed or appropriate.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.