TamperMonkey has a terrible embedded editor. It assumes 4 spaces formatting and some other shit that I don't like, but can't change. I'm also lazy to switch to something else. I just wish they would fix the editor.
Oh the days when user scripts were simply files on your hard disk and could simply be directly edited (and e.g. also globally searched) by the text editor of your choice…
You can actually still use an offline Text editor with TamperMonkey as well (at least with their latest UI, I just checked it)
What? I've been using it for years and that's not true, there's several settings to customise formatting. Have you tried selecting "Advanced" for Config Settings, maybe that's why you can't see those settings.
Simply set "Config mode" to "Advanced" search for the "Editor" section and configure everything as needed.