add small arrows before folders (#427)

lua/nvim-tree/config.lua

@@ -1,7 +1,7 @@
 local M = {}
 
 function M.get_icon_state()
-  local show_icons = vim.g.nvim_tree_show_icons or { git = 1, folders = 1, files = 1 }
+  local show_icons = vim.g.nvim_tree_show_icons or { git = 1, folders = 1, files = 1, folder_arrows = 1 }
   local icons = {
     default = "",
     symlink = "",
@@ -15,6 +15,8 @@ function M.get_icon_state()
       ignored = "◌"
     },
     folder_icons = {
+      arrow_closed = "",
+      arrow_open = "",
       default = "",
       open = "",
       empty = "",
@@ -60,6 +62,7 @@ function M.get_icon_state()
     show_file_icon = show_icons.files == 1 and vim.g.nvim_web_devicons == 1,
     show_folder_icon = show_icons.folders == 1,
     show_git_icon = show_icons.git == 1,
+    show_folder_arrows = show_icons.folder_arrows == 1,
     icons = icons
   }
 end