Compiler shouldn't allow named slots other than at top level inside component

[Conduitry]

Describe the bug
The compiler permits inline components containing named slots that appear inside other elements inside the component. I don't think this means anything or should be allowed. The currently produced code does not behave well.

To Reproduce

<script>
	import Wrapper from './Wrapper.svelte';
</script>

<Wrapper>
	<div>
		<div slot='foo'>
		</div>
	</div>
</Wrapper>

Expected behavior
The compiler throws an exception and refuses to compile this.

Severity
Low

[Conduitry]

In 3.20.0, this is now a compiler error.