Add `ffl` and `tti` ligatures to Mona Sans

Mona Sans currently includes ligatures such as `ff`, `fi`, `fl`, `ffi`, `fy`, `tt`, and `ti`, but it is missing `ffl` and `tti`.

Since `f`+`f`=`ff`, `f`+`l`=`fl`, `t`+`t`=`tt`, `t`+`i`=`ti` and `ff`+`i`=`ffi` already exist, adding `ffl` and `tti` would make the set more consistent and complete.
Example:

Additionally, please make sure that the new ligatures are compatible with **ss01** and **ss03**.

No source files or tests are named in the issue. Start by locating Mona Sans’s glyph and OpenType feature sources and the build entry point, then compare the existing ff, fi, fl, ffi, tt, and ti ligatures. Done means ffl and tti render correctly and remain compatible with ss01 and ss03.

Written by the indexing model from the issue text.