new port: x11/urxvt-font-size

A perl extension for x11/rxvt-unicode that allows changing the
font size on the fly with keyboard shortcuts.

WWW: https://github.com/majutsushi/urxvt-font-size

Reviewed by: mat, jrm (mentor)
Approved by: jrm (mentor)
Differential Revision: https://reviews.freebsd.org/D12463