Fix extension registration to not only check nested properties

for spec compliance, but also the toplevel properties, if any.

stix2/registration.py

@@ -1,3 +1,4 @@
+import itertools
 import re
 
 from . import registry, version
@@ -144,7 +145,12 @@ def _register_extension(
 
     """
     ext_type = new_extension._type
-    properties = new_extension._properties
+
+    # Need to check both toplevel and nested properties
+    prop_groups = [new_extension._properties]
+    if hasattr(new_extension, "_toplevel_properties"):
+        prop_groups.append(new_extension._toplevel_properties)
+    prop_names = itertools.chain.from_iterable(prop_groups)
 
     _validate_type(ext_type, version)
 
@@ -161,7 +167,7 @@ def _register_extension(
                 ext_type,
             )
 
-        for prop_name in properties.keys():
+        for prop_name in prop_names:
             if not re.match(PREFIX_21_REGEX, prop_name):
                 raise ValueError("Property name '%s' must begin with an alpha character." % prop_name)