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.