Add check for ligature patterns

[jordandelbar]

I recently encountered the ligature issue in Zed and found #378.
This PR adds a check to ensure that ligature patterns are not broken.

disclaimer: font rendering and font shaping are not within my domain of competence and I shamelessly vibe coded it + verify that the tests + clippy & examples pass but I am not in capacity to ensure that it does not have other impacts (performance wise, etc) or if this is the correct way to fix this.

Closes #378

In zed

before:

after (built with my branch as source for cosmic-text):

[jackpot51]

This may be okay, but I need to do testing, and there could be a way to fix this without hardcoding ligatures.

[peppidesu]

I don't know if there is a way to retrieve a list of ligatures of a font. This would still leave another issue though: ligatures should be broken when necessary, which this solution prevents from happening.