fix: better children snippet / default slot interop

[dummdidumm]

• correctly assign children snippet to default slot, fixes Snippets and slots interop #13067
• allow svelte:fragment without let: directives to be rendered by @render children(), fixes Invalid parent uses let: error when parent doesn't use let: #13066

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[changeset-bot]

🦋 Changeset detected

Latest commit: 52051b4

The changes in this PR will be included in the next version bump.

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[dummdidumm]

Self-merging because this is rather critical

[hmnd]

Thank you so much! Appreciate the quick turnaround.

[brunnerh]

@dummdidumm Can named slots be made to work as well?
REPL showing all kinds of snippet/slot combinations

[benmccann]

@brunnerh thanks for testing the various combos. Could you file a new issue for that?

[brunnerh]

@benmccann There would already be #13063 which was (probably prematurely) marked as documentation.

[brunnerh]

Added #13784 for better visibility.