Add Conditional for Pro Icons

[MyNameIsCosmo]

The \ifthen condition should be applied to the pro icons.yml in the release.
Currently, the release is only for Font Awesome 5 Free.

Something like:

% freeiconhere
\iffontawesome@pro
% proiconhere
\fi
% freeiconhere

This is also pending some licensing inquiries about the metadata included in the Font Awesome 5 Pro package.