[web] fix clicks on merged semantic nodes (attempt #2) (flutter#47360)

This relands flutter#43620 with a fix for nested tappable nodes. The first PR introduced this regression: flutter/flutter#134842.

This PR includes the original PR and a fix for the regression. The fix is to call `stopPropagation` on the "click" event so that it is not handled by the ancestor if the child has already decided to send a `SemanticsAction.tap` to the framework. This ensures that there cannot be more than one `SemanticsAction.tap` sent to the framework.

Fixes flutter/flutter#134842

Author: yjbanov