This commit moves code in all node editor files to the `blender::ed::space_node` namespace, except for C API functions defined in `ED_node.h`, which can only be moved once all areas calling them are moved to C++. The change is fairly straightforward, I just moved a couple of "ED_" code blocks around to make the namespace more contiguous, and there's the method for adding a pointer to a struct in a C++ namespace in DNA. Differential Revision: https://developer.blender.org/D13871