In previous versions of Rails, a dynamic regex was built to find templates.
After that, PathParser started to be used to both match and sort templates.
With the dynamic regex, templates with lowdash locales (es_AR) were
found properly. But the PathParser regex does not match locales with this
format, only allowing dash (es-AR) or no dash (es). Templates with lowdash
locales have a wrong virtual path and get filtered.

In this commit the PathParser regex is extended to support the lowdash.