feat - add open and close tree events (#1151)

doc/nvim-tree-lua.txt

@@ -988,6 +988,22 @@ on_folder_created({handler})
 on_folder_removed({handler})
                 Registers a handler for when a folder is removed.
 
+                Parameters: ~
+                    {handler}    (function) Handler function, with the
+                                 signature `function(payload)`.
+
+                                   *nvim-tree.events.on_tree_open()*
+on_tree_open({handler})
+                Registers a handler for when NvimTree is opened.
+
+                Parameters: ~
+                    {handler}    (function) Handler function, with the
+                                 signature `function(payload)`.
+
+                                   *nvim-tree.events.on_tree_close()*
+on_tree_close({handler})
+                Registers a handler for when NvimTree is closed.
+
                 Parameters: ~
                     {handler}    (function) Handler function, with the
                                  signature `function(payload)`.