feat(event): add TreeAttachedPost (#1877)

* feat(event): add OnAttachPost

* feat(event): add TreeAttachPost

* feat(event): add TreeAttachPost

* feat(event): TreeAttachedPost fired after all mappings created, not just on_attach

doc/nvim-tree-lua.txt

@@ -1653,6 +1653,12 @@ e.g. handler for node renamed: >
                 handler parameters: ~
                   {folder_name}  `{string}` Absolute path to the removed folder.
 
+- Event.TreeAttachedPost
+                Invoked after the tree's buffer has been created and mappings
+                have been applied: |nvim-tree-mappings| or |nvim-tree.on_attach|
+                handler parameters: ~
+                  {buf}          `{number} `API buffer handle (buffer number)
+
 ==============================================================================
  9. BOOKMARKS                                            *nvim-tree-bookmarks*