71843 Improve the ScrollBar behavior when nested (a: quality, cla: yes, f: cupertino, f: material design, f: scrolling, framework, severe: new feature, waiting for tree to go green)

72308 Add ScrollbarTheme/ScrollbarThemeData (cla: yes, f: material design, f: scrolling, framework, severe: new feature, waiting for tree to go green)

72316 Remove Scrollbar.isAlwaysShown assert based on Scrollbar.controller (a: quality, cla: yes, f: cupertino, f: material design, f: scrolling, framework, severe: new feature, waiting for tree to go green)

72560 Add PrioritizedIntents to support multiple shortcut configurations (a: desktop, cla: yes, f: focus, f: scrolling, framework, platform-web, severe: new feature, waiting for tree to go green)