refactor(icons): do not load web icons if not needed (#989)

We can set show_buffer_icons to false to disable web icons, but the
icons is still being loaded during initialization. This is
costly(loading the icon package needs ~ 20ms while the bufferlin package
needs a few ms init time) and
unnecessary when user doesn't need the icons anyways.

Author: casprwang

lua/bufferline/utils/init.lua

@@ -217,6 +217,9 @@ function M.get_icon(opts)
     if icon then return icon, hl end
   end
 
+  if not config.show_buffer_icons then
+    return "", ""
+  end
   local loaded, webdev_icons = pcall(require, "nvim-web-devicons")
   if opts.directory then
     local hl = loaded and "DevIconDefault" or nil