fix: clear previous_task reference after abort in Tween (#18541)

avoids memory leaks

.changeset/tween-previous-task-leak.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: clear previous_task reference after abort in Tween to prevent memory leak on interrupted tweens

packages/svelte/src/motion/tweened.js

@@ -275,6 +275,7 @@ export class Tween {
 				}
 
 				previous_task?.abort();
+				previous_task = null;
 			}
 
 			const elapsed = now - start;