chore: various playground fixes

[Rich-Harris]

fixes a few annoyances with playgrounds/demo:

• root component is now main.svelte rather than App.svelte, making it much easier to copy test cases into src
• src isn't junked up with entry-client.ts and entry-server.js
• fewer files — no need to have entry-server.js and server.js be separate things

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Latest commit: 7d862e5

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