Haxe 5 Flash target

One strategic decision that I've been avoiding is whether or not to remove the Flash target for Haxe 5. Initially, it seemed obvious to do so because it's an end-of-life technology. It hasn't been on my radar though because the target is working so well that it rarely ever gets in the way of anything. So I think it _could_ be kept around, but should it?

No files or tests are named. Start by reviewing the existing Flash target and the Haxe 5 target roadmap, then determine whether the target should be retained or removed. Done means a documented decision with clear follow-up scope for the chosen direction.

Written by the indexing model from the issue text.