The previous validation for checking if slots with let directives were rendered with `{@render children(...)}` had false positives
- threw an error even if the other side didn't make use of the arguments, i.e. wasn't actually using a let directive
- didn't check that the rendered snippet actually was the children property
This fixes the validation by only applying it to children render tags, and by adding the slot to `$$slots.default` instead of `$$props.children` in more cases (when it's using `<svelte:fragment>` or `let:` directives, which both mean you're using old slot syntax)
Fixes#12414
Simon H