[SuperTextField] - Add tapping WidgetTester extensions and implement missing tests

[matthew-carroll]

When working on #2080 I wanted to duplicate the Super Editor tap behavior to SuperTextField. However, when writing the tests I noticed that we don't seem to have WidgetTester extensions to tap at character offsets, which makes the tests much more complicated.

Add those extensions and then implement the empty tests in super_textfield_ios_selection_test.dart.

Also, make the selection heuristic configurable for SuperTextField similar to how it is for SuperEditor.