add undotree

lua/custom/plugins/undotree.lua

@@ -0,0 +1,8 @@
+return {
+  'jiaoshijie/undotree',
+  dependencies = 'nvim-lua/plenary.nvim',
+  config = true,
+  keys = { -- load the plugin only when using it's keybinding:
+    { '<leader>u', "<cmd>lua require('undotree').toggle()<cr>" },
+  },
+}