Add Widgetbook setup with theme configuration, analysis options, and initial component use cases

packages/button_group_m3e/lib/widgetbook/button_group_m3e_usecases.dart

@@ -0,0 +1 @@
+

packages/loading_indicator_m3e/analysis_options.yaml

@@ -0,0 +1,8 @@
+include: ../../analysis_options.yaml
+
+analyzer:
+  exclude:
+    - "**/*.g.dart"
+    - "**/*.freezed.dart"
+    - "lib/widgetbook/**"
+    - "lib/**/widgetbook/**"

packages/navigation_rail_m3e/lib/src/navigation_rail_m3e_widget.dart

@@ -415,8 +415,12 @@ class _NavigationRailM3EState extends State<NavigationRailM3E>
         for (final dest in section.destinations) {
           final index = _destinationIndex(widget.sections, dest);
           children.add(Padding(
-            padding: const EdgeInsetsDirectional.only(
-                start: 16, end: 16, top: 8.0, bottom: 8.0),
+            padding: EdgeInsetsDirectional.only(
+              start: 16,
+              end: 16,
+              top: theme.itemVerticalGap,
+              bottom: theme.itemVerticalGap,
+            ),
             child: RailItem(
               destination: dest,
               selected: index == widget.selectedIndex,