Factor out AnimatableTile{Factory}

ts/animatable_tile_factory.ts

@@ -0,0 +1,11 @@
+import { TileFactory } from './tile_factory.js';
+
+export abstract class AnimatableTileFactory extends TileFactory {
+  animations: Map<string, [Keyframe[], object]> = new Map();
+
+  add_animation(name: string, keyframes: Keyframe[], options: object) {
+    this.animations.set(name, [keyframes, options]);
+  }
+
+  abstract copy(): AnimatableTileFactory;
+}