Problem/Motivation
Various core features, and contrib, could benefit from a single 'development mode' toggle where certain functionality is enabled or disabled.
Steps to reproduce
Proposed resolution
After a discussion at DrupalCon Barcelona, we decided to add a boolean, and that it should just be 'development mode' or not - e.g. the expectation is that people would set development mode when working locally, but not on dev/staging environments (or if they set it on those environments, then it's an intentional decision). A tri-state flag would be too open to interpretation and could end up undermining itself.
Ideally we want to be able to set this in settings.php/ local services.yaml, but possibly also have a state override for it too so that e.g. drush can be used to set it.
Remaining tasks
Decide where the flag will be implemented.
