fix: warn when using rest or identifier in custom elements without props option

[paoloricciuti]

Sprouted from this bsky conversation

It could be quirky to debug such things so I think a warning could be nice if you have a Rest element or an Identifier as the declarator of $props AND don't have a props option for customElementOptions.

I've added three test that maybe is too much but wanted to capture all three scenarios.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 9c4db99

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16003-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16003

[JetLua]

<!-- A.svelte -->
<svelte:options customElement={{
  tag: 'a-btn',
  props: {
    name: {type: 'String'},
  }
}}/>

<script lang="ts">
  import B from './B.svelte'
  const props: {name: string} = $props()

</script>

<B name={props.name}/>

<!-- B.svelte -->

<script lang="ts">
  const props: {name: string} = $props()
</script>

<button>{props.name}</button>

Perhaps a trigger warning shouldn't be issued for file B.svelte.

https://stackblitz.com/edit/vitejs-vite-1tpdpcqa?file=src%2FB.svelte

[Rich-Harris]

Please open a new issue, comments on merged PRs will get lost