Is it common for supervisors to approach their student and ask to work on publishing the student's masters thesis? My supervisor emailed me asking if he can work on trying to publish my masters thesis, with me being the first author. I have said yes, is it common for supervisors to approach students asking to publish students research?
There are in all likelihood differences between fields of study. In mine, it is uncommon, but if it happend it would be a good sign: You have produced something that is worth publishing, which is rare for a Master thesis in my field. Turning that research in an article that can be published is an art, which I would not expect a master student to master. So having the supervisor help you in that would be necessary in my field. It is right, that your supervisor immediately cleared who would be first author, and also right that you are the first author. That to me is a sign, that (s)he has the best intentions.

When I make such offers, I consider this as offering those students an opportunity to get an advantage very early in their (potential) academic career, or at least to get some inside experience of what working in academia is really like.