Thanks to the oEmbed standard, you can add https://runkit.com/ to the setting allowed iframes and call it a day.
Just paste a runkit URL in a line by itself and Discourse will work it’s magic:
https://runkit.com/falco/5e13a3b7f5d2e2001a5741d2
becomes
It does becomes weird when the embed is too long, so a theme-component to help decorate (and scroll) embeds may be needed indeed.