feat: unevalStream playground

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

Stacked on #218. This is a copy of the #184 playground, adapted to the new unevalStream API so the two implementations can be compared side by side with completely separate playgrounds. Not meant to be merged as-is.

• The worker calls unevalStream synchronously, and the async-iterable tracking is gone from the worker and snapshot code, since feat: unevalStream #218 doesn't stream iterables (yet).
• The default example shows the new model: shared identity across head and tail blocks, delayed promises, a nested promise inside a settled value, a Vector class revived via the js-tag replacer, and a promise folded into head with Promise.resolve(…) from the replacer.
• Repo-level bits are the same as feat: playground #184: playground/* in the workspace, prettier-plugin-svelte and prettier ignores for .svelte-kit/.vercel.

pnpm install
pnpm --dir playground/uneval-stream dev

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}

[changeset-bot]

⚠️ No Changeset found

Latest commit: 6bb3e73

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

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

[pkg-svelte-dev]

Install the latest version of devalue from 6bb3e73:

pnpm add https://pkg.svelte.dev/devalue/c/6bb3e739c88939fdc0c1d842d0ffa1f5dd46a171

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/devalue/pr/219