CodeEditor supports lua on pages in the "Module" namespace.
however, many times you'd like to test your module on a subpage of your user page before publishing it, like you often do with templates and articles. The Extension TemplateSandbox already support testing modules with names from the form "User:myusername/Module:modulename", but when you try to edit a page in that name, you have to do it with the regular wiki editor.
That problem does not exist with js and css pages because they are being identified by their extension, not their namespace.
I think that code editor should be available in modules that are subpages of a user page.
I have also opened a ticket in https://phabricator.wikimedia.org/T217508