Pull request/parallel lua 5.1

Unfortunately github is also showing the #20679, #20680 and #20681 changesets, since this changeset is predicated upon those. Merging them to master in the correct order will do the right thing though, AFAICT.

This changeset is an improved changest to lua-5.1.x Formula, moved to lua51.rb to facilitate parallel installation of all three Lua interpreters used by the majority of Lua developers to ensure portability.

Also tweaked to install lua51.pc so that, at least, highlight.rb can be updated to the latest highlight-3.14 release which looks for a lua-5.1.x pc file at only that location.