fix: handle pointercancel only for in-progress gestures

[maribethb]

The basics

• I validated my changes

The details

Resolves

Fixes #9249 and fixes #9371

Proposed Changes

• Fix: pointercancel event is not handled #9250 fixed Fix not handled pointercancel event #9249 but used too big of a hammer, it would fire pointerup events when scrolling dropdowns. Scrolling a dropdown is something handled by the browser, so it would fire a pointercancel event once it determined that the touch is a scroll instead of a pointerevent the application should handle. This broke dropdowns and possibly other scenarios.
• Reverting Fix: pointercancel event is not handled #9250 alone would reintroduce Fix not handled pointercancel event #9249
• I fixed that by adding an event listener for pointercancel that ends the gesture. This event is only added when a gesture is started.

Reason for Changes

Fixes a regression.

Test Coverage

Tested both bugs using a real touch device. I pushed my changes to github pages so I could test physically. https://maribethb.github.io/blockly/tests/playground.html

Documentation

n/a

Additional Information

This will need to go in a patch release