Queued forward navigations should automatically cancel when executing a backwards navigation.

ex. In the photo gallery example:

1. Start on /photos/2
2. Click Next button which starts a navigation to /photos/3 with pushNewEntry(callback)
3. Navigation event fires and respondWith() starts a 500ms animation
4. Click Next 9 more times queuing 9 callbacks with pushNewEntry(callback)
5. Click Back button taking you to /photos/1
6. No more navigations happen, all 9 callbacks are cancelled.

The expected user behavior is not that the app goes back to /photos/1 and then jumps forward 9 times, but that all pending forward navigations are cancelled whenever going backwards.

I don't think this should be done manually through an AbortSignal. In practice the queued navigations are likely to come out of third party libraries or unrelated components.