Two-way binding to let: var throws compile-time exception

[Conduitry]

<script>
	let Foo;
</script>

<Foo let:bar>
	<input bind:value={bar}>
</Foo>

Throws a compile-time exception, attempting to access a property of something that's undefined.

I haven't put much thought into this, but it seems like two-way binding with something from let: might be difficult or impossible, in which case we should just have a better error here.

[Rich-Harris]

Yeah, I'm gonna go with 'difficult'. I'm sure it's not impossible, but my sense is that it would require some book-keeping that would usually go unused, so I vote for a better error.