Add Type 2 GSUB support for font freezing

[kaushikgopal]

Fixes issue #4

Problem:

• The ss04 "Simplified i" feature uses a Type 2 substitution rule in the font (maps i → ['i.simple'])
• opentype-feature-freezer lib only understood Type 1 substitution rules
• When it encountered Type 2 rules, it silently skipped them
• so customizations like ss04 never got applied to the generated fonts

Solution:

• Remove opentype-feature-freezer dependency (doesn't support Type 2)
• Create fontfreeze_activation.py (custom feature freezing script adapted from https://github.com/MuTsunTsai/fontfreeze)
• Extended it to detect and process Type 2 substitution rules
• Now when ss04 is enabled, it correctly freezes i → i.simple into the font's character map

[philipkbh]

This fixed both the ss04 issue and a weird upstream bug in the opentype-feature-freezer that caused major problems with the instantiate-code-fonts.py script when building a custom Recursive Mono in my NixOS configuration.