NeoVIM: add undo tree plugin

Configs/nvim/lua/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>" },
+    },
+}