feat: Add pause and isPaused to SpriteAnimationTicker (#2660)

Based on the Discord discussion and consultation with @ufrshubham, this PR adds the ability to pause a SpriteAnimationTicker.

packages/flame/lib/src/sprite_animation_ticker.dart

@@ -47,6 +47,16 @@ class SpriteAnimationTicker {
   /// still image).
   bool get isSingleFrame => spriteAnimation.frames.length == 1;
 
+  bool _paused = false;
+
+  /// Returns current value of paused flag.
+  bool get isPaused => _paused;
+
+  /// Sets the given value of paused flag.
+  set paused(bool value) {
+    _paused = value;
+  }
+
   /// A future that will complete when the animation completes.
   ///
   /// An animation is considered to be completed if it reaches its [isLastFrame]
@@ -68,6 +78,7 @@ class SpriteAnimationTicker {
     currentIndex = 0;
     _done = false;
     _started = false;
+    _paused = false;
 
     // Reset completeCompleter if it's already completed
     if (completeCompleter?.isCompleted ?? false) {
@@ -111,9 +122,12 @@ class SpriteAnimationTicker {
   /// calls to [onStart].
   bool _started = false;
 
-  /// Updates this animation, ticking the lifeTime by an amount [dt]
-  /// (in seconds).
+  /// Updates this animation, if not paused, ticking the lifeTime by an amount
+  /// [dt] (in seconds).
   void update(double dt) {
+    if (_paused) {
+      return;
+    }
     clock += dt;
     elapsed += dt;
     if (_done) {

packages/flame/test/sprite_animation_ticker_test.dart

@@ -162,5 +162,26 @@ void main() {
       animationTicker.completed;
       expect(animationTicker.completeCompleter!.isCompleted, false);
     });
+
+    test('paused pauses ticket', () async {
+      final sprite = MockSprite();
+      final animationTicker = SpriteAnimation.spriteList(
+        [sprite, sprite],
+        stepTime: 1,
+        loop: false,
+      ).createTicker();
+
+      expect(animationTicker.isPaused, false);
+      expect(animationTicker.currentIndex, 0);
+      animationTicker.update(1);
+      expect(animationTicker.currentIndex, 1);
+      animationTicker.paused = true;
+      expect(animationTicker.isPaused, true);
+      animationTicker.update(1);
+      expect(animationTicker.currentIndex, 1);
+      animationTicker.reset();
+      expect(animationTicker.currentIndex, 0);
+      expect(animationTicker.isPaused, false);
+    });
   });
 }