Set the text selection to the given range.
The provided argument is a Map<String, int> which includes the keys
extent indicating where the selection within the
value of the
semantics node should start and where it should end. Values for both
keys can range from 0 to length of
extent to the same value will move the cursor to
that position (without selecting anything).
static const SemanticsAction setSelection = SemanticsAction._(_kSetSelectionIndex)