fix: allow svelte:self in snippets

[paoloricciuti]

This closes #9436 however i'm not entirely sure we should do this.

On one hand is reasonable to be able to render <svelte:self /> inside a snippet that is than passed to a component even without an if block. On the other this remove a safe security pin against infinite loops if i do something like

{#snippet infinite()}
<svelte:self />
{/snippet}

{@render infinite()}

at the same time this is still doable by doing this

{#snippet infinite()}
    {#if true}
        <svelte:self />
    {/if}
{/snippet}

{@render infinite()}

For the moment i've opened this PR but i leave this to the wise minds of the maintainers to decide what's best to do.

P.s. i did not include a test to check for this behavior because i didn't see tests to check that svelte:self is allowed inside the other blocks....but i can add one for all the cases if needed.

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[dummdidumm]

Makes sense to me to allow it like this. Static analysis can only get you so far when preventing infinite loops.

[Rich-Harris]

nice, thanks