Improve and document JSON output

[Kobzol]

This PR changes the JSON output based on feedback from #241.

What has changed:

1. JSON output should now always produce parseable JSON. Notably, it no longer outputs multiple lines with JSON, but always a single JSON value.
2. Errors are now printed in a printer-specific way (in JSON they are outputted as a dictionary with an error key).
3. Most JSON outputs were reworked and their output was manually specified. Dates and durations are now formatted correctly and enumerations are also formatted in a more reasonable way.
4. Tests were rewritten. They now use structural matching of the output vs an expected schema (using the schema library). They still lack a lot of situations to test though.

Documentation is missing currently. I want to document some basic JSON schema of the output (however not everything will be stabilized right away).

Fixes: #241