Add a public read-only parent property to TreeNode

See #1397.

tests/tree/test_tree_node_parent.py

@@ -0,0 +1,10 @@
+from textual.widgets import TreeNode, Tree
+
+def test_tree_node_parent() -> None:
+    """It should be possible to access a TreeNode's parent."""
+    tree = Tree[None]("Anakin")
+    child = tree.root.add("Leia")
+    grandchild = child.add("Ben")
+    assert tree.root.parent is None
+    assert grandchild.parent == child
+    assert child.parent == tree.root