This is really awesome. Great job!
One improvement that I would like is to make it installable via Nimble. From what I can tell I currently need to clone this repo and put my arduino project inside it, then call make to build my project. It would be much better if I could nimble install arduimesp and then run arduimesp build inside my project, you can trivially create a program that replicates the work of your makefiles (and it should be more portable as a bonus).