fix: overwrite #last_value when Spring is `.set()` with `{instant: true}` (#14656)

* fix: overwrite #last_value when Spring is  with

* run npx changeset

.changeset/shy-carpets-rescue.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+Overwrite Spring.#last_value when using .set() with {instant: true}

packages/svelte/src/motion/spring.js

@@ -282,6 +282,7 @@ export class Spring {
 			this.#task?.abort();
 			this.#task = null;
 			set(this.#current, set(this.#target, value));
+			this.#last_value = value;
 			return Promise.resolve();
 		}