Babylon has cyos for making glsl shaders, but its usefulness is limited by the fact that the inputs to the shader usually end up different than what CYOS provides. So while CYOS can be used as a starting off point for development of a shader, there comes a point when you need to integrate your shader into your project and use different attributes/uniforms, and after that you can’t really go back to CYOS. Also it’s beneficial to develop a shader on real scene geometry and not just a test mesh.
What would useful is to have a source editor as part of the debug inspector. So that you can click on a ShaderMaterial and edit its source code(s) with live updating as you type. It needn’t be fully featured, something like codemirror would be fine.
I’ve set this up in my own project and it makes writing shaders a lot easier.