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/m3e_design/lib/utils/build_context_x.dart

@@ -0,0 +1,7 @@
+import 'package:flutter/material.dart';
+import '../theme/m3e_theme.dart';
+
+extension BuildContextM3EX on BuildContext {
+  M3ETheme get m3e =>
+      Theme.of(this).extension<M3ETheme>() ?? M3ETheme.defaults(Theme.of(this).colorScheme);
+}

packages/m3e_design/lib/utils/semantics_x.dart

@@ -0,0 +1,5 @@
+import 'package:flutter/material.dart';
+
+extension SemanticsX on Widget {
+  Widget withLabel(String label) => Semantics(label: label, child: this);
+}