allow endpoints to be treated as prerender entry points

[Rich-Harris]

fixes #2541. An endpoint like src/routes/index.js or src/routes/sitemap.xml.js or whatever should be prerenderable.

Though now that I've implemented it and I'm writing this comment, it occurs to me that this should probably only apply to endpoints that are explicitly marked as prerenderable, unless config.kit.prerender.default === true. Will leave this open pending #4093.

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[Rich-Harris]

obsolete since #6392