JS: Include content properties in type-tracker properties

Reminder: we have two PropertyName classes because the one in Contents.qll can't depend on DataFlow::Node.

javascript/ql/lib/semmle/javascript/dataflow/internal/StepSummary.qll

@@ -1,6 +1,7 @@
 import javascript
 private import semmle.javascript.dataflow.TypeTracking
 private import semmle.javascript.internal.CachedStages
+private import semmle.javascript.dataflow.internal.Contents as Contents
 private import sharedlib.SummaryTypeTracker as SummaryTypeTracker
 private import FlowSteps
 
@@ -30,6 +31,8 @@ private module Cached {
         SharedTypeTrackingStep::loadStoreStep(_, _, _, this)
         or
         this = DataFlow::PseudoProperties::arrayLikeElement()
+        or
+        this instanceof Contents::Private::PropertyName
       }
     }