react-router Add support for fetcher.abort()

This has primarily 2 use cases:

When using v7_fetcherPersist, we lost the ability to actually cancel/abort fetches if a fetcher was unmounted, so this gives the users a programmatic API to achieve the previous behavior if necessary
When "streaming" data via the user of promises, once the fetcher has returned to "idle" there's no good way to abort/cancel the ongoing streaming data since we only tracked the controller in the router while the fetcher was in a non-idle state. This keeps those controllers around inside the router until the fetcher is deleted so that they can be programatically aborted after returning to idle

When calling abortFetcher, we simply abort the controller which will proxy through to the request.signal in the loader so you can use that to reject the promises and trigger the <Await errorElement>:

function loader({ request }) {
  return {
    critical: "CRITICAL",
    lazy: new Promise((resolve, reject) => {
      request.signal.addEventListener("abort", () =>
        reject(request.signal.reason)
      );
      setTimeout(() => resolve("LAZY"), 3000);
    }),
  };
}