[adapter-vercel] Make all root routes be called index instead of an empty string

[Schniz]

The current implementation of edge: true, split: true will fail to deploy on Vercel because the Edge Function name is empty. This PR fixes that by renaming them to index instead.

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Latest commit: 7f6ff20

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@sveltejs/adapter-vercel	Patch

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[Rich-Harris]

thanks! it's actually way simpler — a route ID never ends with / (i.e. there's no possibility of separate nested and nested/ routes), so the only case we need to consider is the empty string