Better haxe compile integration from mix workflow

[fullofcaffeine]

The user shouldn't need to compile manually, the changes should be detected at the Haxe source by a watcher, and then they should be incrementally compiled by Haxe. Investigate.

[fullofcaffeine]

Source map support too, as part of this. Also, research if this component would be ideally in Haxe or the target language (elixir) -- check other reflaxe / haxe targets and investigate later. Could start with Elixir to make things simpler and to more easily integrate with mix.