This change adds support for triple click to select a paragraph at the clicked position and triple click + drag to extend the selection paragraph-by-paragraph when using the SelectionArea widget.
This PR also:
* Makes `Text` widgets a `SelectionContainer` if a parent `SelectionRegistrar` exists.
* Fixes issues with selectable ordering involving `WidgetSpan`s.
Fixes: https://github.com/flutter/flutter/issues/104552