Add initial configuration, tokens, and widgets for M3E components

- Introduced `.gitignore` and `.metadata` for apps and examples.
- Added Flutter/Dart analysis configurations (`analysis_options.yaml`).
- Implemented foundational tokens and themes for M3E (colors, shapes).
- Created base implementations for `IconButtonM3E` and `SplitButtonM3E`.
- Set up CI workflow (`ci.yaml`) to automate testing and analysis.

packages/button_group_m3e/test/button_group_m3e_test.dart

@@ -0,0 +1,7 @@
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('placeholder', () {
+    expect(1 + 2, 3);
+  });
+}