Add a pg::link utility

[dimo414]

TIL (some) terminal emulators now have support for hyperlinks via escape sequences: https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda

It's called out in https://en.wikipedia.org/wiki/ANSI_escape_code#OSC_(Operating_System_Command)_sequences but it's easy to miss. My understanding is unsupported terminals will simply not display the link.