Tidy neovim configuration

* Remove some unused plugins and packages.
  * e.g. floaterm, projectionist, vim-wiki, vim-easy-align,
    vim-surround, visual-star-search, vim-rhubarb
* Replace none-ls with conform.nvim and nvim-lint.
* Replace lualine.nvim with mini-statusline.
* Add neodev.
* Simplify cmp completion sources.

after/plugin/completion.lua

@@ -26,13 +26,9 @@ cmp.setup {
   },
 
   sources = {
-    { name = "nvim_lsp_signature_help" },
     { name = "nvim_lsp" },
     { name = "nvim_lua" },
-    { name = "cmp_tabnine" },
-    { name = "path" },
     { name = "luasnip" },
-    { name = "buffer", keyword_length = 5, max_item_count = 5 },
   },
 
   sorting = {