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/tokens/spacing_tokens.dart

@@ -0,0 +1,37 @@
+import 'package:flutter/material.dart';
+
+@immutable
+class M3ESpacing {
+  final double xs; // 4
+  final double sm; // 8
+  final double md; // 12
+  final double lg; // 16
+  final double xl; // 24
+  final double xxl; // 32
+
+  const M3ESpacing({
+    required this.xs,
+    required this.sm,
+    required this.md,
+    required this.lg,
+    required this.xl,
+    required this.xxl,
+  });
+
+  const M3ESpacing.regular()
+      : xs = 4,
+        sm = 8,
+        md = 12,
+        lg = 16,
+        xl = 24,
+        xxl = 32;
+
+  static M3ESpacing lerp(M3ESpacing a, M3ESpacing b, double t) => M3ESpacing(
+        xs: a.xs + (b.xs - a.xs) * t,
+        sm: a.sm + (b.sm - a.sm) * t,
+        md: a.md + (b.md - a.md) * t,
+        lg: a.lg + (b.lg - a.lg) * t,
+        xl: a.xl + (b.xl - a.xl) * t,
+        xxl: a.xxl + (b.xxl - a.xxl) * t,
+      );
+}