Change TreeNode.toggle to act only from the source node

See #1644 (comment)
where Darren raises the excellent point that while the "technically correct"
approach that I had was... well, technically correct I guess (it toggled all
the nodes from the target node down), it didn't have what was likely the
desired effect.

So this commit does away with the previous logic for doing the toggle and
instead simply calls on expand or collapse depending on the state of the
source node.

src/textual/widgets/_tree.py

@@ -201,26 +201,16 @@ def collapse(self, *, collapse_all: bool = False) -> None:
         self._collapse(collapse_all)
         self._tree._invalidate()
 
-    def _toggle(self, toggle_all: bool) -> None:
-        """Toggle the expanded state of the node.
-
-        Args:
-            toggle_all: If `True` toggle all offspring at all depths.
-        """
-        self._expanded = not self._expanded
-        if toggle_all:
-            for child in self.children:
-                child._toggle(toggle_all)
-        self._updates += 1
-
     def toggle(self, *, toggle_all: bool = False) -> None:
         """Toggle the expanded state.
 
         Args:
             toggle_all: If `True` toggle all offspring at all depths.
         """
-        self._toggle(toggle_all)
-        self._tree._invalidate()
+        if self._expanded:
+            self.collapse(collapse_all=toggle_all)
+        else:
+            self.expand(expand_all=toggle_all)
 
     @property
     def label(self) -> TextType: