I really would appreciate such a shader editor. It doesn't need to be a graphical node editor (in fact I like plain programming more

), but syntax highlighting, a list of built-in functions/auto-completion, quick help on parameters and, like everwind proposed, a way to preview the shader as a map would be very helpful with developing shaders.
Many others mentioned the potential of ISL (and galinette recently proved it), but it is still too much time-comsuming trial and error when using it.