configitems: register the 'extensions' section

mercurial/configitems.py

@@ -412,6 +412,10 b" coreconfigitem('experimental', 'treemani"
 coreconfigitem('experimental', 'updatecheck',
     default=None,
 )
+coreconfigitem('extensions', '.*',
+    default=None,
+    generic=True,
+)
 coreconfigitem('format', 'aggressivemergedeltas',
     default=False,
 )