Fix dependency problem

require 'nvim-web-devicons' fails if colors.setup done in plugin/. This
may be related to loading order.

lua/nvim-tree.lua

@@ -215,6 +215,7 @@ function M.xdg_open()
   end))
 end
 
+colors.setup()
 vim.defer_fn(M.on_enter, 1)
 
 return M