[feat] allow +server.js files next to +page files

[dummdidumm]

Closes #5896

Also added a related paragraph to "Form Actions" because I saw some people asking about this specifically.

Marking as draft because I don't know what to make of this TODO:

kit/packages/kit/src/core/adapt/builder.js

Line 49 in 55493cb

type: route.page ? 'page' : 'endpoint', // TODO change this if support pages+endpoints

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[dummdidumm]

aside: it's strange to me that this is private API - it's public because it's part of the public builder methods