Add optional isSelected toggle state support to ButtonStyleButton #194202

Use case

In Material 3 and Material 3 Expressive (see #168813, #184936, #184937), common buttons (FilledButton, FilledButton.tonal, ElevatedButton, OutlinedButton, TextButton) and custom ButtonStyleButton subclasses support toggleable selected/unselected states (updating WidgetState.selected for color/shape resolution and setting Semantics.selected for accessibility).

Currently, ButtonStyleButton does not expose an isSelected parameter. As a result:

1. Even inside Flutter's own Material library (icon_button.dart), _IconButtonM3 extends ButtonStyleButton has to be wrapped in a separate 100-line _SelectableIconButton extends StatefulWidget solely to allocate a WidgetStatesController, synchronize WidgetState.selected in initState/didUpdateWidget, and wrap the child in Semantics(selected: isSelected).
2. Every custom ButtonStyleButton subclass or toggleable Material button must duplicate that boilerplate StatefulWidget wrapper to manage WidgetStatesController lifecycle and Semantics(selected: ...).

Proposal

Add an optional bool? isSelected property directly to ButtonStyleButton:

• In _ButtonStyleState.initStatesController(), if widget.isSelected != null, call statesController.update(WidgetState.selected, widget.isSelected!).
• In _ButtonStyleState.didUpdateWidget(), if widget.isSelected != oldWidget.isSelected, call statesController.update(WidgetState.selected, widget.isSelected ?? false).
• In _ButtonStyleState.build(), pass selected: widget.isSelected to the outer Semantics widget.
• Optionally simplify _IconButtonM3 in icon_button.dart by passing isSelected directly to ButtonStyleButton and removing _SelectableIconButton.