add since tag to Spring (#14585)

Author: Rich-Harris

packages/svelte/src/motion/public.d.ts

@@ -34,6 +34,7 @@ export interface Spring<T> extends Readable<T> {
  * <input type="range" bind:value={spring.target} />
  * <input type="range" bind:value={spring.current} disabled />
  * ```
+ * @since 5.8.0
  */
 export class Spring<T> {
 	constructor(value: T, options?: SpringOpts);