create separate functions for ISR routes

[Rich-Harris]

Follow-up to #9182 — we disable streaming mode for ISR routes, but we don't consider config.isr when splitting functions in the first place. This means that if you have an SSR route and an ISR route that share the same config, one of two things will happen, depending on the order in which they're encountered during build:

• the SSR route won't use streaming
• the ISR route will 'fall back' to SSR

Both are undesirable. This fixes it

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@sveltejs/adapter-vercel	Patch

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[Rich-Harris]

gonna do a cheeky self-merge so that this demo can be updated