nvim: Add undotree plugin

Lua implementation of the undo tree plugin, mapped to be
reachable with <leader>su (for show undo).

nvim/.config/nvim/lua/plugins/ui.lua

@@ -50,4 +50,11 @@ return {
 		end,
 		event = "VeryLazy",
 	},
+	{
+		"jiaoshijie/undotree",
+		dependencies = {
+			"nvim-lua/plenary.nvim",
+		},
+		config = true,		event = "VeryLazy",
+	},
 }