static of(fn, options) {
		const spring = new Spring(fn(), options);

		render_effect(() => {
			spring.set(fn());
		});

		return spring;
	}
Create a spring whose value is bound to the return value of `fn`. This must be called
inside an effect root (for example, during component initialisation).

```svelte
<script>
	import { Spring } from 'svelte/motion';

	let { number } = $props();

	const spring = Spring.of(() => number);
</script>
```
@template U
@param {() => U} fn
@param {SpringOpts} [options]