I was working on my first simple plugin today using the editor at Design → Custom Themes and thought that plugins may not be able to have a settings screen unless they are distributed through the directory because I wasn’t seeing a settings screen for my plugin (though the values stored in the default config were working fine and the plugin was showing under Plug-ins).
Turns out I had a trailing comma in my JSON in plugin.json, but there was no indication in the UI or logs that there was an issue 
It would be helpful to have linting or another form of feedback for JSON files to help catch those kinds of errors 
