Use Unicode word boundaries with the stdlib regexp engine
Go's regexp only treats ASCII characters as word characters for \b, while
regexp2 uses Unicode ones, so switching the builtin path pattern to the
stdlib engine changed which paths were marked next to non-ASCII text, for
example config.yaml文件 marked config.yaml. Match against a proxy of the
text in which every non-ASCII rune is replaced by a single ASCII byte that
is a word character iff the rune is one, then map the offsets back.