kakoune: support for adding plugins

Motivation: There is a thriving plugin ecosystem for Kakoune now,
and it is nice to add these in our Nix configurations. This was modeled
on neovim's plugins.

parinfer-rust is useable both standalone and as a Kakoune plugin,
so the plugin file inherits the same definition as pkgs.

I'll make PRs for other plugins if this gets accepted.
[Here](https://github.com/eraserhd/nixpkgs/tree/kak-ansi)'s a tested
branch for the `kak-ansi` plugin.