fix: improve types for props when using Client-side component API

[ivanhofer]

fix for https://github.com/sveltejs/language-tools/issues/1578

In theory this is a breaking change. So maybe it needs to wait for version 5 of Svelte.

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[ivanhofer]

Ah, didn't knew about this issue. I updated it to use null instead.

[benmccann]

The types are more restrictive, but you wouldn't have been able to pass something that doesn't conform to these types without the code at least breaking at runtime I think? So maybe it's not really a breaking change?