Is anyone aware of a word processor, text editor, whatever, that can handle HTML definition lists natively?
But you're editing markdown, not HTML. And, for example, markdown doesn't have definition lists.
That said, prosemirror's got a pretty steep learning curve.
For a definition list, you’d probably want two node types: a definition list, and a definition list item.
And then create toDOM and parseDOM methods to render and parse respectively the html you want.