Allow highlights to have accessories

[StevenEWright]

[x] I would be willing to contribute this feature to the project if given guidance and support.

Is your feature request related to a problem? Please describe.

The editor doesn't expose any features that make it easy to present error or warning summaries to the user, but it does have highlights; which sort of beg for this other feature.

Describe the solution you'd like

I'd like to present a short summary of errors and warning messages to users in the same line as the error/warning in the same way Xcode would.

Like this:

Describe alternatives you've considered

What I'd really love is a feature that allows users to specify the height of some expandable space below each line, and a feature to populate that space with client-supplied content. This would allow for a lot more inline error and warning text, especially in width-constrained environments, and would be more flexible in general. But this would be a lot more work and I'm not sure I have the time for it right now, and the two features aren't mutually exclusive either, so implementing the one doesn't rule out implementing the other too at a later date.

Additional context

N/A