feat: add idPrefix to render

[Rich-Harris]

Alternative to #15403. Rather than adding idPrefix to mount and hydrate, it makes client-side ID generation global via the window.__svelte object. This is more reliable than expecting people to deconflict in the rare cases that multiple Svelte runtimes exist on the page.

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Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 57b4d3a

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

This PR includes changesets to release 1 package

Name	Type
svelte	Minor

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15428-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15428

[Hugos68]

This is awesome! @Rich-Harris, @paoloricciuti and @dummdidumm thanks for helping me with this and coming up with a solution quickly. I can now submit a PR to Astro to include it in their sveltejs integration so that islands will have unique ID's!

[Rich-Harris]

Thank you @Hugos68!