Add config.kit.prerender.origin, enable fetching from local endpoints during prerendering

[Rich-Harris]

Fixes #5444

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Latest commit: 37affe5

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[dummdidumm]

Two minor questions, but nothing that blocks me from approving this

[dummdidumm]

This will make all globals writable - could this have unintended side effects, like people starting to monkey patch this stuff when they shouldn't? Should we limit this to fetch, or is it actually better that people have the ability to override if needed (I'd say yes, but I wanted to bring this up).

[Rich-Harris]

Yeah, I had the same internal conversation. I decided that since we're able to write these values, it's only fair that others can too — there are always going to be legitimate reasons why someone might want to intercept one of these things (if only for debugging)

[dummdidumm]

Should we make it more apparent that this is from the $page store and/or link to it?

Suggested change

	- `origin` — the value of `url.origin` during prerendering; useful if it is included in rendered content
	- `origin` — the value of [`$page.url.origin`](/docs/modules#$app-stores-page) during prerendering; useful if it is included in rendered content

[Rich-Harris]

The trouble is it's not just $page, it's the url property of RequestEvent and LoadEvent too, so I figured url.origin was the most catch-all. Most people will never need to touch this setting anyway so it probably doesn't matter all that much