refactor: let's clean the config for Xmas (#1)

requires neovim/neovim@7121983c45d92349a6532f32dcde9f425e30781e or later

## changelog
- Lua
  - add a Lua "editor config"
  - set Lua's "tab width" to 2
  - remove useless descriptions and comments
  - format Lua code
- move Nushell NUON filetype to `after/plugin/nushell.lua`
- update Lazy bootstrap snippet
- plugins
- simplify completions with
[`blink.cmp`](https://github.com/Saghen/blink.cmp)
  - simplify LSP with Lua, Nushell and Rust
- simplify Telescope with ivy theme, more vim-motions, simpler keymap
and multigrep
  - add an LSP-based "_format on save_"
  - remove which-key entirely (i don't use it)
  - remove Neogit
  - remove vim-flog
  - simplify gitsigns keymaps
- add bindings for terminals in all directions as well as a stateful
floating one
- keymaps
  - Aerial is now under `<leader>a`
  - gitsigns is now under `<leader>g`
  - LSP is now under `<leader>l`
- update lock file
- add a custom mouse popup menu