Remove default scrollbars from Container

As per #2361 this removes default scrollbars on Container (and so adds them
on ScrollableContainer).

src/textual/containers.py

@@ -18,7 +18,7 @@ class Container(Widget):
     Container {
         height: 1fr;
         layout: vertical;
-        overflow: auto;
+        overflow: hidden hidden;
     }
     """
 
@@ -26,6 +26,13 @@ class Container(Widget):
 class ScrollableContainer(Widget, inherit_bindings=False):
     """Base container widget that binds navigation keys for scrolling."""
 
+    DEFAULT_CSS = """
+    ScrollableContainer {
+        layout: vertical;
+        overflow: auto auto;
+    }
+    """
+
     BINDINGS: ClassVar[list[BindingType]] = [
         Binding("up", "scroll_up", "Scroll Up", show=False),
         Binding("down", "scroll_down", "Scroll Down", show=False),