chore: unify sandbox

[Rich-Harris]

We have two things in playgrounds — demo and sandbox. demo is for testing out how things run, sandbox is for testing out how things compile.

This distinction makes no real sense, and I often find myself editing the wrong file and then being confused as to why nothing is changing. I don't see any reason not to combine them both into a single sandbox directory.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

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

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