Add code reformatting/indent to REPL

[rlaferla]

PROBLEM:
I use the REPL frequently and perhaps my sveltejs/svelte-repl#1 complaint is that the indentation gets out of whack after a while and the code becomes unreadable. I then have to copy the code to a local editor, reformat/indent it and then paste it back.

SOLUTION:
Add a button that reformats/reindents the code to prettify it.

IDEAS:
Perhaps, the code in this post could be used as a starting point:

https://stackoverflow.com/questions/26360414/javascript-how-to-correct-indentation-in-html-string