feat(runtime): undotree #35627

Problem
No builtin way to visualize and navigate the undo-tree.

Solution
Include an "opt" plugin.

runtime/pack/dist/opt/nvim.undotree/plugin/undotree.lua

@@ -0,0 +1,8 @@
+if vim.g.loaded_undotree_plugin ~= nil then
+  return
+end
+vim.g.loaded_undotree_plugin = true
+
+vim.api.nvim_create_user_command('Undotree', function()
+  require 'undotree'.open()
+end, {})