Extensions: add BLENDER_USER_EXTENSIONS user directory

Replace: `{BLENDER_RESOURCE_PATH_USER}/extensions`
With:    `{BLENDER_USER_EXTENSIONS}`

This follows BLENDER_USER_CONFIG & BLENDER_USER_SCRIPTS conventions.
Reading the environment variable and accessible via
`bpy.utils.user_resource('SCRIPTS')`

source/blender/python/intern/bpy.cc

@@ -223,6 +223,7 @@ static PyObject *bpy_user_resource(PyObject * /*self*/, PyObject *args, PyObject
       {BLENDER_USER_DATAFILES, "DATAFILES"},
       {BLENDER_USER_CONFIG, "CONFIG"},
       {BLENDER_USER_SCRIPTS, "SCRIPTS"},
+      {BLENDER_USER_EXTENSIONS, "EXTENSIONS"},
       {0, nullptr},
   };
   PyC_StringEnum type = {type_items};