As someone who used Scripts - Background A LOT, This one just makes me giddy with excitement. We finally have a first class editor with all the features you expect (except saving, of course.) Formatting, commenting, search, LINE NUMBERS, intellisense/introspection, error indicators, color coded keywords, the thumbnail scroller (or whatever the official name is for that thing) on the right. I love it!
If you're not familiar, Scripts - Background lets you run server-side code on-demand. This carries a risk - This is not recommended for production instances, but can be a real time saver in dev if you need to reset fields across records, test triggering events, Flow actions, etc.
You can find it by navigating to All > System Definition > Scripts - Background.
Tip: Delete and recreate bookmarks you may have already created or you'll be disappointed. If you use the one under System Definition and look at the URL, you'll notice that it goes to sys.scripts.modern.do. However, if you have an existing bookmark, it will still navigate to sys.scripts.do giving you the old UI.
And yes, we know there are wonderful third party add-ons like Xplore and SNUtils. These are wonderful tools and you are welcome to continue using them. My initial tests with SNUtils shows that it doesn't conflict with the new editor. My guess is that the new editor is using a different id in the DOM that it doesn't try to modify. This may change in the future once @Arnoud Kooi gets his hands on Washington D.C. 🙂