I use Atom (https://atom.io/), but I don't remember if .cocoascript had syntax highlighting by default, or if I had to enable it. Either way, it's possible.
The plugins I create reside in the Sketch Plugins directory, and always show me my latest/greatest changes due to the following terminal command, which is a tip found on the Sketch Developer - Scripting Preferences page (http://developer.sketchapp.com/introduction/preferences/)...
defaults write ~/Library/Preferences/com.bohemiancoding.sketch3.plist AlwaysReloadScript -bool YES
When debugging, open up Console.app and monitor feedback in ~/Library/Logs > com.bohemiancoding.sketch3 > Plugin Output.log
Hope that's enough to get you started.