Component refs and exported properties

[dkzlv]

Is your feature request related to a problem? Please describe.
I am not really sure if it is a feature request, bug report or request for docs.

Svelte allows you to use exported const properties when using component refs.

Child.svelte:

<script lang="ts">
  export const get = () => 'hey';
</script>

Parent.svelte:

<script lang="ts">
  import Child from './Child.svelte';

  let childRef: Child | undefined = undefined;
  
  // Error: `Property 'get' does not exist on type 'Child__SvelteComponent_'.` when calling the function
  const call = () => childRef!.get();
</script>

<Child bind:this={childRef} />

Describe the solution you'd like
It would be cool if the compiler knew the signatures of exported properties automatically.

Describe alternatives you've considered
I see two alternatives, both of which drop the typing security drastically:

1. use any for refs
2. assign the type explicitly

let childRef: Child & { get: () => string }

It's bad, because when child changes its signature parent would never know about it until we manually update the signature.

[dummdidumm]

Duplicate of #307