Svelte 5: Minimalest upgrade

[jamesst20]

• Upgraded playground to Svelte 5
• Fixed SSR breaking changes
• packages.json: Allow Svelte 5 installation
• Improvements: Render CSS properly for Svelte 3 & 4 include component CSS in head #1761
• Svelte 5 conventions used: None

Breaking change

Svelte 5 changed how SSR rendering works. Under Svelte 3&4, you can call MyComponent.render.
Under Svelte 5 you must import the render method from a package that doesn't exist under Svelte 3 & 4 making impossible to have compatibility for both unless we extract the component rendering outside.

We must now pass an extra parameter function that will handle the rendering outside Inertia giving the user the opportunity to use any Svelte version.

// ssr.js
createInertiaApp({
    ....
    ssr: (AppSSR, props) => {
      // Svelte 4: return AppSSR.render(props)
      // Svelte 5: return render(AppSSR, { props })
      return render(AppSSR, { props })
    },
})

I have added a warning

if (!ssr) {
      throw new Error(`createInertiaApp must provide ssr(...) for server-side rendering.`)
    }

Warning to Svelte 5 users

• You still need to use $ syntax when importing reactive state from Inertia such as $page
• Super reactive state will not work! Meaning $form.something.push({...}) will not trigger state refresh. This is likely a really huge bummer so you may consider using my fork adapter instead @jamesst20/inertia-svelte5. Hopefully Inertia maintainers change their about having a separate Svelte 5 adapter.
• You still need to do stuff like $form.something = $form.something to trigger state refresh on updated object without creating a full copy before updating even if you use Svelte 5