Add syntax highlighting

[flimzy]

@samlown found https://ace.c9.io/, which is a syntax-highlighting code editor in JS, although it's perhaps more heavy-weight than we need, and it requires a pretty big overhaul of how we handle text input. And it's really only suited for one of the three windows we offer (the input window). Still not out of the question.

In searching, I found this post that offers another option for syntax highlighting: https://css-tricks.com/creating-an-editable-textarea-that-supports-syntax-highlighted-code/

Some more investigation is probably warranted to determine the best way to add syntax highlighting to the input and output windows (and maybe the JWT window).

[samlown]

Ace does look rather large and complex... we're not just after highlighting though, we need text-editor like features that help you write YAML or JSON documents. Even basic things like pressing "enter" and the next line being aligned with the previous is a big plus in experience. Check this one out for another example: https://jsonformatter.org/json-viewer. There must be an open option somewhere!

[samlown]

This is now solved thanks to the Monaco editor.