[neovim] Give key bindings a little love; add GUI bindings for neovide

- Clean up the init interface. Move all the init methods into a single
  init_all() call.
- Add two keybindings for <M-j> and <M-k> to move by soft-wrapped lines in
  Normal mode
- Add a few key bindings for the usual shortcuts for cut/copy/paste when in GUI
  mode. In vim, D is the character that represents the Super/Apple key. So,
  <D-x>, <D-c>, and <D-v> now do what you'd expect.

config/nvim/init.lua

@@ -56,10 +56,7 @@ require 'treesitter'
 require 'lsp'
 
 local keys = require 'keys'
-keys.init_key_opts()
-keys.init_window_key_mappings()
-keys.init_diagnostic_key_mappings()
-keys.init_telescope_mappings()
+keys.init()
 
 local gui = require 'gui'
 gui.init()