Existing features solve this problem already. They just need to be used.
The problem is comments containing answers preventing the Q&A format from working right. The question assumes the solution is adding a feature to promote a comment to an answer, when existing features do the job plenty.
We don't have this problem at RPG.se because we recognise comments that contain answers are broken windows. So, we aggressively flag and delete answers in comments.
They're not comments, they're answers. If someone wants to contribute an answer, they can scroll down a half-screen and use the correct input field. If they couldn't be bothered to do that, then they knew they didn't actually have a worthwhile answer for some reason, and we can use that helpful meta-information to categorise their comment as noise that the page would be better without.
The problem with comments in answers is many-fold, not least of which is that they don't receive the expected peer-review attention of the voting system. By deleting them, we force anyone who wants to actually contribute an answer to put in the work to write something that they actually have confidence in enough to stake their reputation on, or to keep their peace.
The result? We don't have
broken windows answers in comments lying everywhere, the voting system is given the grist it requires, questions that have answers actually register in the system as having answers, and people can find quality help that much easier.