reasons:
- they are two lexicons with lots of overlapping fields we should maintain
- I think, and wish we will have more comment targets, like commenting on commit like GitHub currently supports.
I'm aware of sh.tangled.pull.comment might have more fields in future, mostly related to reviewing. But considering how many fields both lexicons share, I think merging both as a single general "comment" lexicon and "review comment" inheriting it might make sense.