Current modes are `LOG` and `LISTING`, which `log()` and store for conversion to json respectively.
Add `ContentListing` listing struct to (recursively) contain help data for conversion to a json object to be exported and used elsewhere (e.g. the docs).
Rather than formatting as rst we can just export with type information and do the conversion at the destination (i.e. in the python code which loads the domain for autodoc).
Implement `PrettyHelp::has_content()`.
Provide `PrettyHelp::get_content()` which returns a read-only list of the current content.
`PrettyHelp` constructor takes optional `Mode` enum to define format of help content.
Updates `PrettyHelp` methods to use a switch case for checking current mode, calling `log_abort()` in the default case (i.e. unsupported mode).
d4498acea7