Add Tree.get_node_by_id

The thinking here is that a user of a Tree may want to relate notes to other
parts of their UI, or other data in their application. While this could be
done by keeping a reference to the node itself, it could also be beneficial
to just track the ID. Given that ID is a public property of a TreeNode, but
given it doesn't currently have any other (public) purpose, this seems to
add some useful symmetry.

tests/tree/test_tree_get_node_by_id.py

@@ -0,0 +1,16 @@
+import pytest
+from typing import cast
+from textual.widgets import Tree
+from textual.widgets._tree import NodeID
+
+
+def test_get_tree_node_by_id() -> None:
+    """It should be possible to get a TreeNode by its ID."""
+    tree = Tree[None]("Anakin")
+    child = tree.root.add("Leia")
+    grandchild = child.add("Ben")
+    assert tree.get_node_by_id(tree.root.id).id == tree.root.id
+    assert tree.get_node_by_id(child.id).id == child.id
+    assert tree.get_node_by_id(grandchild.id).id == grandchild.id
+    with pytest.raises(KeyError):
+        tree.get_node_by_id(cast(NodeID, grandchild.id + 1000))