Parses and consumes a <group>.
The productions are: <group> -> “(”<regex>“)”
“(?:”<regex>“)”
“(?>”<regex>“)”
“(?<flags>:”<regex>“)”
“(?=”<regex>“)”
“(?!”<regex>“)”
“(?<=”<regex>“)”
“(?<!”<regex>“)”
“(?(”<num>“)”<regex>“)”
“(?(”<regex>“)”<regex>“)”
“(?<name>”<regex>“)” (when ALLOW-NAMED-REGISTERS is T)
<legal-token>
where <flags> is parsed by the lexer function MAYBE-PARSE-FLAGS.
Will return <parse-tree> or (<grouping-type> <parse-tree>) where
<grouping-type> is one of six keywords - see source for details.