fix: Throw on unrendered snippets in dev

[elliott-with-the-longest-name-on-github]

Currently, if you try to render a snippet without @render, Svelte will not complain -- it'll just stringify the snippet to the DOM. That's pretty annoying. During dev, we'll now throw an error reminding you to render your snippets.

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[changeset-bot]

🦋 Changeset detected

Latest commit: b7b6991

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15766

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15766-svelte.vercel.app/

[paoloricciuti]

Wait wasn't #15521 fixing this?

[paoloricciuti]

Oh no just looked at the code right now and it's a different situation

[dummdidumm]

looks good apart from the error message