Factor out AnimatableTile{Factory}

ts/animatable_tile.ts

@@ -0,0 +1,19 @@
+import { Tile } from './tile.js';
+
+export abstract class AnimatableTile extends Tile {
+  animations: Map<string, [Keyframe[], object]>;
+
+  constructor(width: number, height: number, animations: Map<string, [Keyframe[], object]>) {
+    super(width, height);
+    this.animations = animations;
+  }
+
+  play(name: string): Animation | undefined {
+    const animation = this.animations.get(name);
+    if (animation) {
+      return this.elem.animate(...animation);
+    } else {
+      return undefined;
+    }
+  }
+}