Export Spring interface

[razshare]

Is your feature request related to a problem? Please describe.
I'm trying to export spring stores but I can't specify their types as Spring<...> for the external binding variables.

Describe the solution you'd like
Can the Spring interface be exported?

svelte/src/runtime/motion/spring.ts

Lines 61 to 67 in ce3a579

	interface Spring<T> extends Readable<T>{
	set: (new_value: T, opts?: SpringUpdateOpts) => Promise<void>;
	update: (fn: Updater<T>, opts?: SpringUpdateOpts) => Promise<void>;
	precision: number;
	damping: number;
	stiffness: number;
	}

Use case

<!-- file: welcome.svelte -->
<!-- some unrelated markup -->
<div 
    class="circle"
    style="
        --x:{$x}px;
        --y:{$y}px;
    "
>r</div>
<!-- some more unrelated markup -->
<style>
.circle{
    --x:0;
    --y:0;
    top:var(--y);
    left:var(--x);
}
</style>
<script lang="ts">
//...
import { spring } from 'svelte/motion';

export let x = spring(0,{
    stiffness: 0.1,
    damping: 0.25
});
export let y = spring(0,{
    stiffness: 0.1,
    damping: 0.25
});
//...
onMount(()=>{
    document.addEventListener("mousemove",e=>{
        //some logic

        x.set(/*some value*/);
        y.set(/*some value*/);
    });
})
</script>

Being able to do this and specify types would be useful:

<!-- file: home.svelte -->
<Welcome bind:x bind:y/>
<script lang="ts">
import type { Spring } from 'svelte/motion';    // <=== this would be useful
import Welcome from './welcome.svelte';

let x:Spring<number>;
let y:Spring<number>;

//now I know for sure that x and y are Spring<number>
</script>

---

Same thing with Tweened:

svelte/src/runtime/motion/tweened.ts

Lines 66 to 70 in ce3a579

	interface Tweened<T> extends Readable<T> {
	set(value: T, opts?: Options<T>): Promise<void>;
	update(updater: Updater<T>, opts?: Options<T>): Promise<void>;
	}