Just a word of warning. Designer does a ton more than just syntax highlighting, including syntax validation as you type. The arcane nature OH exceptions means that debugging simple syntax errors like unmatched curly brackets or a typo in an Item name can take hours to debug whereas Designer will tell you immediately. It also can code completion with <ctrl><space> which helps one discover/remember the methods that are on various Items saving one the need to look things up.
So my suggestions would be, if you are serious about using an editor outside of Designer, to see if you can recreate these capabilities. Without that you might be setting yourself up for frustration in the future. Maybe keep Designer around as a debugging tool (i.e. when it doesn’t work load everything into Designer to check everything for syntax errors).