add config.kit.endpointExtensions option

[Rich-Harris]

Alternative to #4185 — prevents files other than .js and .ts files from being treated as endpoints, unless you specify endpointExtensions

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[Conduitry]

I do feel a bit better about this than the other option. Do we want to rename the existing extensions option to pageExtensions or something similar? Or move both under a single object?

[Rich-Harris]

The wrinkle there is that config.extensions is used by other tooling, like mdsvex

[Conduitry]

Oh. Hm. And it lives outside of kit. Never mind, I don't have a good suggestion about this then.