re-organize nvim lua init

[davidjenni]

• leverage lazy-vim's best practice of listing individual plugin config lua files under nvim/lua/plugins and let lazy-vim merge those; see: https://github.com/folke/lazy.nvim#-structuring-your-plugins
• flatten nvim config folders
• add tree sitter support for better syntax coloring
• add LSP config for lua (requires lua-language-server to be installed)
• add which-key to show possible key bindings after first typed cmd key
• install additional LSP language support via :Mason