Is there a GLSL editing mode for modern NetBeans (v8+)?
There are plugins but for older versions. For example a syntax highlighter: http://plugins.netbeans.org/plugin/46515/glsl-syntax-highlighter.
But if I were you I’d just edit your shaders in something like Notepad++.
I’m using that plugin for GLSL support in Praxis LIVE, which is based on NetBeans 8.0, so you should be alright there. At some point I’ll look at extending the somewhat limited feature set, and will either contribute back or possibly release as a fork.