Add formula for ltl2ba

Adds formula for `ltl2ba`. This is a dependency for Frama-C, but can also be used separately.

I've based the PR off #8215 but updated to use the newer homebrew APIs. This is also my first Homebrew PR, so it's possible that I've missed something :panda_face: 

/cc @mikemcquaid
Nitpick, but can you use double-quotes everywhere?