chore: add more sensible defaults for icons

adds a symlink and default icon by default
fixes #1192

lua/nvim-tree/renderer/icon-config.lua

@@ -3,8 +3,8 @@ local M = {}
 function M.get_config()
   local show_icons = vim.g.nvim_tree_show_icons or { git = 1, folders = 1, files = 1, folder_arrows = 1 }
   local icons = {
-    default = "",
-    symlink = "",
+    default = "",
+    symlink = "",
     git_icons = {
       unstaged = "✗",
       staged = "✓",