Notifies the engine when `PointerSignalEvents` have been ignored by the framework, through the `ui.PointerData.respond` method.
This allows the web to "preventDefault" (or not) on `wheel` events.
## Issues
* Fixes (partially): https://github.com/flutter/flutter/issues/139263
## Tests
* Added tests to ensure `respond` is called at the right time, with the right value.
## Demo
* https://dit-multiview-scroll.web.app
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## Previous versions
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1. Modified `PointerScrollEvent`, not shippable.
2. Modified when events were handled, instead of the opposite.
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