Drat. Some years ago (! time sure does fly) I've created a custom one... with collapsable rules and states... Now I'm too lazy to do it again.
You can use mine, if u wish (see attachment). I, too, am using Notepad++ for all of my scripting. Would be really great to have a powerful IDE like Visual Studio with IntelliSense and stop and resume debugging capabilities, but I fear N++ is as good as it gets for Nexus scripting.
Regarding the attached XML: you can import it in Notepad++ under "Languages"->"Define your language"->"Import..."
You might have to change the colors to better fit your default color scheme. I'm using Notepad++'s "Ruby Blue" style with white text on a dark blue background and the syntax highlighting colors for my Nexus Script XML have been adjusted to nicely fit into that color scheme. If you're using black text on white background per default, you will probably have to change those text colors.
I probably also missed some keywords or operators, but so far it works fine for my typical use case.