er... for an idie game be able to edit maps should be great (even better if players can share homemade campaign)
For me in steamwork = bad idea. I will not dowload it for sure and it seem obvious that lot's of people interested by an editor like me will not search to dowload him in standalone. BUT it's best to focus on the main game, release him well done and LATER in a major update add an ingame editor with a place/site/itchio-page where share campaigns.
On an other hand, having the editor ingame to the first release simply make him better, the question are does it take a lot of time to make a clear editor ? if the answer are yes but that you are able to wait a little longer, so do it. Otherwise in an later update seem the best idea.