Allow spring/tweened values to be initially undefined

[Rich-Harris]

Is your feature request related to a problem? Please describe.
When using springs and tweens to describe values that are set from props, I find myself doing this sort of thing:

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

  export let big = false;
  
  const size = spring(big ? 100 : 20);
  $: size.set(big ? 100 : 20);
</script>

In this situation I want the value of size to be static when the component is first created, until the prop changes. But it's a nuisance to have to specify big ? 100 : 20 twice. You could create a function for it...

const get_size = big => big ? 100 : 20;
const size = spring(get_size(big));
$: size.set(get_size(big));

...but that's arguably worse.

Describe the solution you'd like
If it were possible to call spring without an initial value, we could initialise it later:

const size = spring();
$: size.set(big ? 100 : 20);

Of course, it would be nice if we could initialise the store right there in the reactive declaration, as happens for non-store values, but I'm not sure what that would look like. Might be cool to be able to do this sort of thing...

$: <spring>$size = big ? 100 : 20;

...but it's obviously invalid JS. Maybe worth coming back round to that one day, but for the meantime allowing undefined/null initial values solves the DRY problem.

How important is this feature to you?
I'm encountering this situation a lot in my current project. I think it's important that using motion be as frictionless as possible.