Smart colon height adjustment OT feature

SF has some 'liga' code which uses GSUB classes to only apply between `{0-9}{0-9}:{0-9}{0-9}` (excuse the pseudo regex ;) and maybe it is worthwhile to do something similar here to automatically raise the colon to look centered for clock times.

Possibly this should also go in the `case` feature for `{A-Z}:{\ }` etc

Start by locating the existing 'liga' feature code that uses GSUB classes, then inspect how the `case` feature is defined. Determine the intended clock-time patterns and whether uppercase-colon-space cases are included. Done should mean the colon is automatically raised for the agreed patterns and the feature behavior is verified.

Written by the indexing model from the issue text.