Update to svelte 5

[jdtjenkins]

Changes

Updates the @astrojs/svelte integration to use Svelte 5 by default. This change is necessary because the Svelte 5 Vite plugin only supports Svelte 5 and has dropped all v4 support. So it'd be really messy supporting both versions in one integration. We'd be better off cutting a new major and only supporting v5 on the new major and beyond.

Removes all Svelte 4 code and deps from the integration.

Fixes or updates all the tests to use Svelte 5 components.

Testing

All the unit tests have been updated to use the newer Svelte 5 syntax, all the fixtures have also been updated to use Svelte 5 as a dependency. Running the fixtures shows the integration works and loads up Svelte 5 components.

Docs

I... Should add some docs for this yes. This will be a pretty big change(? or a reasonably sized one I guess) as we'll now stick Svelte 4 on a certain major of the package, and carrying on will only be Svelte 5 support.

withastro/docs#10002

[changeset-bot]

🦋 Changeset detected

Latest commit: 732176f

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

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[bluwy]

@jdtjenkins do you mind if I commit directly to bring it up to date? I'd like to get Svelte 5 support ready soon so it's easier to migrate later for the Astro beta.

[jdtjenkins]

@bluwy yeah please do! Sorry I haven't had another chance to look at this yet. But I agree, getting this out asap is best.

If you need me to do anything, let me know and sorry this was more broken than intended!

[bluwy]

No worries! It was already a headstart having the tests updated to Svelte 5. Thanks for the initial work!

[codspeed-hq]

CodSpeed Performance Report

Merging #12364 will not alter performance

_{Comparing update-to-svelte-5 (732176f) with main (3b3bc9b)}

Summary

✅ 1 untouched benchmarks

[bluwy]

Looks like it's passing now, however I still want to look into cleaning up createClassComponent and removing the custom svelte.config.js handling, which should drastically make the integration simpler.

[sarah11918]

Thanks @jdtjenkins and @bluwy - just a quick clarifying question!

[sarah11918]

Approving for docs! (Even if disapproving of the unfortunate numbering alignment. Boooo)