Content is now added to the `ContentListing` rather than the `PrettyHelp`.
`open_*` methods return the `ContentListing` that was added instead of leaving a hanging continuation.
This allows for (e.g.) options to be added directly to optiongroups, instead of requiring that groups be closed before continuation.
This also means that all `PrettyHelp`s are a listing, with the actual log being called by the default `Pass::help()`; making the mode field redundant.
Added `PrettyHelp::log_help()` which replaces the `PrettyHelp::Mode::LOG` logic.
Added `ContentListing::back()` which just returns the last element of the underlying content vector.
Some of the content tracking was made redundant and removed, in particular `PrettyHelp::_current_listing` and `ContentListing::parent`.
Converted `ContentListing` to a class instead of a struct, adjusting constructors to match.
Added `ContentListing` constructor that accepts a `source_location`.
Update `HelpPass::dump_cmds_json()` for new log_help.
1529d991fd