Cleanup: Move node editor files to proper namespace

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

source/blender/editors/space_node/node_select.cc

@@ -62,7 +62,7 @@
 
 #include "node_intern.hh" /* own include */
 
-using blender::float2;
+namespace blender::ed::space_node {
 
 /**
  * Function to detect if there is a visible view3d that uses workbench in texture mode.
@@ -1309,3 +1309,5 @@ void NODE_OT_find_node(wmOperatorType *ot)
 }
 
 /** \} */
+
+}