[new] Support literate Djot files

[nmccarty]

Djot is a markup format that is derived from CommonMark, it's code block syntax is identical to CommonMark's, so we can support it as a CommonMark style literate formate by just adding the file extension.

Description

Should this change go in the CHANGELOG?

• If this is a fix, user-facing change, a compiler change, or a new paper
  implementation, I have updated CHANGELOG_NEXT.md (and potentially also
  CONTRIBUTORS.md).

[buzden]

Could you, please, add a test for this case?

[nmccarty]

I have copied literate011, which appears to be the smoke test for the .md extension (interestingly there's no such test for the .markdown extension, though there is still at least on literate that checks that the compiler at least acknowledges the existence of .markdown files) and just changed the extension to .dj