fix: allow fetch in remote function without emitting a warning

[Rich-Harris]

Ordinarily if a server-side fetch occurs outside a load function, we emit a warning that steers people towards using event.fetch instead. This makes sense in the old world, where fetches in universal loads are serialized, but not really in the new world — we want people to use regular fetch inside remote functions, because the return value of the function itself is the thing that gets serialized.

I also suspect that the is_in_remote_function state will come in handy in various ways.

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