Previously, ripgrep would always defer to the regex engine's capturing
matches in order to implement word matching. Namely, ripgrep would
determine the correct match offsets via a capturing group, since the
word regex is itself generated from the user supplied regex.
Unfortunately, the regex engine's capturing mode is still fairly slow,
so this commit adds a fast path to avoid capturing mode in the vast
majority of cases. See comments in the code for details.
cd8ec38a68