fix: change url protocol for prerender

[bluwy]

Fixes #3164

Fore prerenders, change fallback sveltekit protocol to http. (Thoughts?)

In Node and Safari, the sveltekit protocol is recognized and properly parsed. Firefox and Chrome however don't and incorrectly parses it. e.g. new URL('sveltekit://prerender/'), Firefox and Chrome reported //prerender/ as the pathname.

Looking at the spec (the scheme table), I think Node and Safari are correct, since presumably we're not using a domain or ipv4 address as the host.

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[Rich-Harris]

I think this is indicative of a deeper bug that resulted from #3126 — before, we'd pass a path (which is the same whether you're prerendering or not) but now we need a full URL for hydration. (The obvious thing to do would be to just use new URL(location), but that has edge cases of its own when you're using Kit to prerender snippets of HTML that are then embedded on a different path.)

This seems like a workable fix for now but we should probably think of something more comprehensive