Svelte 5: Allow @const inside an snippet block

[kauderk]

Describe the problem

Svelte 5 preview introduced {#snippet ... } {/snippet} block but previous functionality like the {@const} tag is missing. You'll get the following error:

{@const} must be the immediate child of {#if}, {:else if}, {:else}, {#each}, {:then}, {:catch}, <svelte:fragment> or <Component>

Describe the proposed solution

Follow up with this issue #7241

Alternatives considered

None

Importance

would make my life easier

[rmunn]

I suggest putting "Svelte 5:" at the start of the issue title so the maintainers will more immediately notice that it's Svelte 5-related.