Rewrite Neovim config

[tomodachi94]

The current configuration is a mess:

• Some of the configuration was "glue-code" for my non-NixOS system, and now it either conflicts or is redundant on NixOS. Examples: d713b7e and b64f05e
• Configuration for plugins is spread out vastly and inconsistently, causing the code to be inconsistent with the style of the other parts of the repo.
• Much of the code was copy-pasted from other sources, leading to a very inconsistent code style. (I don't even remember what some of it does!)
  • Some mysterious Vimscript is floating around, and I don't know what it does...
• Various breakages, such as nvim: Tab not working in command bar #6

Tasks

• Use Neovim nightly
  • We can then drop plugin/ftdetect/mcmeta.lua
• Add quality control measures (linting and testing)
• Identify and add plugins missing from Nixpkgs
• Devise a new organizational scheme for organizing plugins: one per file? One per 'concern'? One big file?
• Port over non-plugin configuration:
  • util.lua
  • options.lua
  • keybinds.lua
• Rewrite "plugin modules" to use lz.n, or drop them if they won't be missed:
  • autocompletion.lua
  • autoinsert.lua
  • extras_movement.lua
  • lsp.lua
  • mason.lua
  • notifications.lua
  • snippets.lua
  • status_bar.lua
  • support_editorconfig.lua
  • support_elixir.lua
  • support_mediawiki_editor.lua
  • syntax.lua
  • thanks.lua
  • theme.lua
  • tools.lua