Fix #227: switch deprecated isAlwaysShown to thumbVisibility

Author: michaelspiss

example/lib/main.dart

@@ -47,7 +47,7 @@ class FontAwesomeGalleryHomeState extends State<FontAwesomeGalleryHome> {
     return Scaffold(
       appBar: _isSearching ? _searchBar(context) : _titleBar(),
       body: Scrollbar(
-        isAlwaysShown: kIsWeb,
+        thumbVisibility: kIsWeb,
         child: GridView.builder(
           itemCount: filteredIcons.length,
           gridDelegate: const SliverGridDelegateWithMaxCrossAxisExtent(