Modernize Vim completion and linting setup

[klappradla]

Changes

• Replace deoplete with nvim-cmp

  Set up basic completion (no LSP yet) with nvim-cmp.

• Set up basic linting with nvim-lint

  NOTE: this throws errors when the specific linters are not available.

• Use null-ls for linting and formatting

  This also sets up some autocommands. For now, I find it easier to only use null-ls for formatting. Otherwise, Vim will always prompt and ask which LSP to use for formatting…

• Some cleanup in lua files

  • format files with stylua
  • lint files with luacheck
  • add .luacheckrc file to define vim as global variable (keeps the noise down)

[klappradla]

Turns out the ultimate source of slowness in Neovim was ultisnips 🐌 See https://github.com/SirVer/ultisnips/issues/1496