[Request]: Replace new setTemplate API with render in defineMeta

[JReinhold]

With the release of Svelte 5.5.0, Svelte introduced "exportable snippets": https://svelte.dev/docs/svelte/snippet#Exporting-snippets

This essentially means that snippets can now be referenced from the module context, versus before where they could only be referenced from the instance context. Given that defineMeta() must always be used in the module context, this means that it's now possible to pass template snippets to defineMeta.

Proposal

This allows us to get rid of the annoying setTemplate() API, that is a shortcut for setting the a default template across all stories in a file. I dislike it for multiple reasons:

1. It requires you to add an instance-script, something that is rarely needed in stories file in general, so it's added verbosity.
2. Conceptually is does the same thing as Storybook's core render function on the meta-level, but it's doesn't look remotely like it.

Here is the API change that I propose:

<script module>
- import { defineMeta, setTemplate } from '@storybook/addon-svelte-csf';
+ import { defineMeta } from '@storybook/addon-svelte-csf';
  import MyComponent from './MyComponent.svelte';

  const { Story } = defineMeta({
    /* ... */
+   render: template
  });
</script>

-<script>
-  setTemplate(template);
-</script>

{#snippet template(args)}
  <MyComponent {...args}>
    ...
  </MyComponent>
{/snippet}

<Story name="With Default Template" />

Rationale

Swapping setTemplate for a meta-level render makes the API almost exactly like regular CSF. This makes knowledge more transferable for users coming from other frameworks, and makes docs, tutorials, articles, etc. more useful in Svelte CSF.

Implementation

A template snippet can't be the actual render property of a story context, because the Svelte CSF sets the internal StoryRenderer component to that.

1. But we should be able to pass it as a prop to StoryRenderer from within createRuntimeStories which has access to the meta.
   https://github.com/storybookjs/addon-svelte-csf/blob/next/src/runtime/create-runtime-stories.ts/#L54-L61
2. I don't think we need to remove the render from the meta, because all Svelte CSF stories have a render defined as per 1, which overrides anything in meta as per the inheritance principle