This will allow adding developer checks which will be fully compiled out in
user builds, unlike `DEBUG_ENABLED` which is included in debug tempates and
the editor builds.
This define is not used yet, but we'll soon add code that uses it, and change
some existing `DEBUG_ENABLED` checks to be performed only in dev builds.
Related to https://github.com/godotengine/godot-proposals/issues/3371.
e292d79fb3
