Process the label on construction of a TreeNode

Currently there's an asymmetry in how the label is handled for a TreeNode.
If a str label is passed to the constructor it stays as a str type. On the
other hand, if it's set via set_label, it gets processed into a Rich Text
type. This commit removes that asymmetry.

src/textual/widgets/_tree.py

@@ -71,7 +71,7 @@ class TreeNode(Generic[TreeDataType]):
         self._tree = tree
         self._parent = parent
         self._id = id
-        self._label = label
+        self._label = tree.process_label(label)
         self.data = data
         self._expanded = expanded
         self._children: list[TreeNode] = []