style: Add parsing support for ::part().

Disabled for now of course. This should be pretty uncontroversial I'd think.

Differential Revision: https://phabricator.services.mozilla.com/D28060

components/malloc_size_of/lib.rs

@@ -747,6 +747,7 @@ where
             Component::ExplicitUniversalType |
             Component::LocalName(..) |
             Component::ID(..) |
+            Component::Part(..) |
             Component::Class(..) |
             Component::AttributeInNoNamespaceExists { .. } |
             Component::AttributeInNoNamespace { .. } |