Units, errors and type display

[pfitzseb]

@ChrisRackauckas

But yes, there must be some easier way to parse this. I really like the Juno tree displays and would like to see them as part of the Base display system, but of course they wouldn’t be usable from the REPL (I think?)

This should actually be quite simple to get working in the REPL as well, afaict. I’m prototyping something based on TerminalMenus.jl right now, which is part of the REPL stdlib anyways.

Sneak peak:

Still a couple of things to figure out, but seems like a good idea overall imho.