Simplify

# General/Nix - [x] Remove all tools that I don't use daily or actively, - [x] Reduce globals & use `nix run` for one-offs, and add project-specific tools to each project flake. - [x] Merge `aerc.nix` and `mail.nix` and get rid of neomutt. - [ ] ~Split things by features even more, if I'm going to use devcontainers I want to be able to just pull `shell.nix` and `vim.nix` for example only.~ # neovim - [ ] 3 git plugins? `Fugitive`, `DiffView` and `mini.diff` (maybe `mini.git`)? Think of JJ support too. - [x] AI plugins, do I need them with CLI tools? Doesn't make much sense. - [x] Review `ftplugin` files