Reduce code duplication between kern writers

[madig]

The two kern writers share a lot of code:

• https://github.com/googlefonts/ufo2ft/blob/main/Lib/ufo2ft/featureWriters/kernFeatureWriter.py
• https://github.com/googlefonts/ufo2ft/blob/main/Lib/ufo2ft/featureWriters/kernFeatureWriter2.py

They should be refactored to extract the commonalities.

[anthrotype]

@rsheeter asks in #962

if possible (are kern 1/2 allowed to share?) it would be nice to see code sharing as this looks duplicative. But perhaps they are specifically supposed to be isolated from one another...?

I forgot the details, maybe @madig can chime in

[madig]

I think they should share code as far as possible. I think I tried to refactor the code a bit when making the new writer, but in the back and forth to get it in, finding limitations and going half-way back, it lead to the two writers diverging...