chore: more slot/snippet interop

This builds on top of #10800 and explores what it takes to make slots and snippets interoperable. It allows you to use render tags already when your consumers still pass slots, or pass snippets already when the component you pass it to still uses slots. From my guess the latter is likely more common (you're depending on some third party library which hasn't updated yet).

Implementation-wise it passes { $$slots: { foo: true } } for a snippet prop named foo, and the render functions for snippets/slots were adjusted to check both locations for content, and react accordingly.

The drawback is that it's a bit more runtime code, and it's somewhat brittle, because it will only work for render tags with 0 or 1 argument (<slot {foo} {bar} /> === @render children({foo, bar})) which could be a bit confusing. It's very nice that it "just works" in the common case though.

Putting this up for completeness even if it's unlikely that we land this.

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[x] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
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Tests and linting

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sveltejs / svelte

chore: more slot/snippet interop #11619

Before submitting the PR, please make sure you do the following

Tests and linting

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