Add compile/v2 returning errors in JSON

[terezka]

Quick Summary:

Endpoint /compile is unchanged. The new endpoint /compile/v2:

In case of successful compile: Returns resulting html for iframe, including a message out to notify editor of success.
In case of unsuccessful compile: Returns html for iframe which sends out the compilation problems (in JSON format) to editor.

This change was made in order to allow more advanced interactions with errors for the elm-lang.org/try page.

[github-actions]

Thanks for suggesting these code changes. To set expectations:

• Pull requests are reviewed in batches, so it can take some time to get a response.
• Smaller pull requests are easier to review. To fix nine typos, nine specific issues will always go faster than one big one. Learn why here.
• Reviewers may not know as much as you about certain situations, so add links to supporting evidence for important claims, especially regarding standards for CSS, HTTP, URI, etc.

Finally, please be patient with the core team. They are trying their best with limited resources.