[makeinstancesufo] allow public.skipExportGlyphs in instances

[arialcrime]

I understand that it does not make sense for all common/public lib keys to be copied to instances, but it seems overly strict (and slightly misleading) to get rid of everything apart from public.glyphOrder and public.postscriptNames in clearCustomLibs.

Allowing public.skipExportGlyphs in instances would also be very handy for this potential future update 🤞 to kern and mark feature writers.

[frankrolf]

Indeed, it would probably make sense to keep all public. lib keys.