Nicer code editor in the playground

[torhovland]

Is your feature request related to a problem? Please describe.
The playground currently uses simple textareas. This could be improved with something that looks more like a code editor.

Describe the solution you'd like
A nice code editor looking playground.

If a parse error refers to line and column numbers, these should be easy to find in the input code.

Describe alternatives you've considered
The current solution is a simpler alternative that works.

Additional context
See https://nickel-lang.org/playground and https://jsfiddle.net for inspiration.