devel/srell: [NEW PORT] Std::regex-like regular expression template library for C++

SRELL (std::regex-like library) is a Unicode-aware regular expression
template library for C++.

WWW: https://www.akenotsuki.com/misc/srell/en/

Approved by: yuri@ (Mentor)

Differential Revision: https://reviews.freebsd.org/D52521