[5/?] Add ability to run code examples in the playground: Run code samples inline

[shaedrich]

See #340

As discussed in #205

[!WARNING] Will only work, after #544 and #542 have been merged. Works in combination with ponylang/pony-playground#212.

Example: https://deploy-preview-554--pony-tutorial.netlify.app/getting-started/hello-world#the-code

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