[web] ignore pointer events on plain text spans

[yjbanov]

A semantic node may be tappable without having a more concrete role set on it, such as "button". It will just have a tap handler. This could lead to the sized span to be chosen as the label representation. However, when pointer events land on the span the DOM target becomes the span rather than the tappable element, and that breaks the debouncing logic in pointer_binding.dart.

This PR removes pointer event handling from inert text spans. This fixes the click debounce logic.

Fixes TBD (looking for an issue to link to)

[yjbanov]

Text selection is done in the framework. Semantics should be unrelated.