Add support for GSUB Lookup Type 3

Add support for the lookup type 3, Alternate Substitution when handling GSUB:

https://learn.microsoft.com/en-us/typography/opentype/spec/gsub#AS

The first available substitution glyph can be used (as done in other libraries)

Also, the current implementation of CompoundCharacterTokenizer doesn't account for collision in ligatures

For example, if a font supports ligatures for att and en, the current implementation will not tokenize properly for the word attention. This is because the regex implementation doesn't allow for a proper split

I'll open a proposed implementation for the above