[breaking] strip __data.json from url

[dummdidumm]

This is breaking change for people who relied on the __data.json suffix being present on event.url which is now no longer the case. Use the new isDataRequest boolean instead.

export function handle({ event, resolve }) {
-  if (event.url.pathname.endsWith('__data.json') {
+  if (event.isDataRequest) {
    // ..
  }
  return resolve(event);
}

closes #7926

Open question: Should we also remove those things from the original request object? I vote for "no".

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[Rich-Harris]

this would prevent contrived-but-allowed things like /__data.json/wheee, no? Don't we just want to check the end of the pathname?

[Conduitry]

This is just part of a test, not the code that users are going to run.

[Rich-Harris]

i definitely knew that and wasn't just being absent-minded