While looking into #12425 (comment) I realised that hydrate inside a programmatic snippet currently fails, because we don't reset hydrate_node.

This fixes it, and also adds a test showing how to render components in programmatic snippets. (It's fine to call hydrate with an empty target — it defers immediately to mount.)

It did force a couple of realisations:

• render(Component, {...}) fails in the client — obviously — because it tries to pass the server $$payload to the client component function that expects an $$anchor. I wonder if render should instead return a { head: '', body: '' } object if it's run in the browser? It would be convenient for this use case, though perhaps surprising for others. (In an ideal world each component would include both client and server code, and we'd somehow treeshake away the stuff we weren't using, but it's not really possible AFAICT.)
• it might be convenient to have a svelte/environment module with dev and browser exports, just like we have in SvelteKit (and in future, it could replace $app/environment). I tried writing the test with a typeof window check, but it turns out window is defined in the Vitest environment so it didn't work, and I had to add a mechanism for adding server props to tests instead

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