devel/tla+: new port

TLA+ is a language for modeling software above the code level and
hardware above the circuit level. The tool most commonly used by
engineers is the TLC model checker, but there is also a proof
checker. TLA+ is based on mathematics and does not resemble any
programming language. Most engineers will find PlusCal to be the
easiest way to start using TLA+.

The graphical TLA Toolbox IDE is not included. It is a separate
Eclipse RCP application that would be better as its own port.

WWW: https://www.tlapl.us

Approved by: dch (mentor)
Reviewed by: arrowd
Pull Request: https://github.com/freebsd/freebsd-ports/pull/554