Add a {#render} block

[adiguba]

Describe the problem

Svelte 4 had a simple syntax to define the slot default code :

<slot>
    <!-- Default code if slot is missing -->
</slot>

Svelte 5 doesn't really have an equivalent for that, and this has to be done manually :

Either via an if/else :

{#if children}
    {@render children()}
{:else}
    <!-- Default code if children is missing -->
{:if}

Or via a second snippet :

{#snippet default_children()}
    <!-- Default code if children is missing -->
{/snippet}
{@render (children??default_children)()}

Describe the proposed solution

It would be nice to have a {#render} block that would mimic the <slot/> behavior :

{#render children()}
    <!-- Default code if children is missing -->
{/render}]

I think that this change may only impact the compiler, as the result code could simply use $.snippet() with a condition :

$.snippet(node, () => $$props.children ?? ($$anchor) => {
	$.next();
	var text_1 = $.text("DEFAULT CHILDREN");
	$.append($$anchor, text_1);
});

If the idea is accepted, I think I could try to implement it...

Importance

nice to have

[brunnerh]

I never found the slot syntax to be very intuitive.
Why would fallback content be the hierarchical child?
It looks like some content is being set but it's a replacement that might be there or not.

This might be more terse, but I am not sure it is actually better.