classify
Classifies the delimiter run starting at an index.
A run that can neither open nor close is still returned, with both flags false, so that the caller consumes it whole as text: re-entering the scan inside a run would classify its remainder against the wrong neighbours, and a__b's second underscore would look like an opener.
Return
the classified run, or null when char is not the character at index
Parameters
source
text to inspect
index
index the run must start at
char
the run's character