Enable stylistic set in IBM Plex Mono

IBM has monospace font that looks good called Plex Mono. which turns out we can enable some stylistic set such as a, g, and slashed zero

To enable opentype feature use this library called opentype feature freezer https://github.com/twardoch/fonttools-opentype-feature-freezer.

we can run the script like so: pyftfeatfreeze.py -n -v -f ‘ss02,ss03,ss04,ss05’ IBMPlex-Mono.otf out/IBMPlex-Mono-ss.otf