75230 [Api docs] Added Vertical Divider dartpad demo (cla: yes, d: api docs, d: examples, documentation, f: material design, framework, waiting for tree to go green)

75761 Added scrollbar example (cla: yes, d: api docs, d: examples, documentation, f: material design, f: scrolling, framework, waiting for tree to go green)

76387 Refactor tabbar code sample (cla: yes, d: api docs, documentation, f: material design, framework, waiting for tree to go green)

77107 Improve error messaging and documentation for Scrollbar.isAlwaysShown (a: error message, cla: yes, d: api docs, d: examples, documentation, f: scrolling, framework, waiting for tree to go green)