After trying the Mark Text demo (very nice), I got very curious about the editor control and how I could integrate it into something I am thinking about working on.

It looks like the editor is called Muya: https://github.com/marktext/muya

And according to this issue: https://github.com/marktext/muya/issues/6

The best way to get access to the latest version of the control is to use the source here: https://github.com/marktext/marktext/tree/develop/src/muya