fix: relax Component type

[dummdidumm]

The current type narrows the binding type to "" by default, which means "no bindings on this component". While this is the common case, it makes it very cumbersome to use the Component type because legacy components are of type string and as soon as you have bindings, the type is something like "foo" | "bar" which also is not assignable to "" which is semantically wrong, because you should be able to assign a component that can have bindings to a type that accepts none. The pragmatic solution is to change the binding type to be string by default, which means someone theoretically could use bindings with a component that doesn't have bindings:

<script>
  let component: Component<{ prop: boolean }> = IAcceptNoBindings;
</script>
<!-- allowed but should be a type error -->
<svelte:component this={component} bind:prop={foo} />

But this is a) rare anyway and b) can be caught at runtime

This came up in comments of #11775

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