LibJS: Skip allocation of temp object for primitive types in Value::get

Previously, `String.prototype.split()` caused the construction of a
temporary StringObject when a string primitive was passed as an
argument, solely to perform a Symbol.split lookup. This change allows
skipping that allocation by looking directly into the prototype of
primitive values.

As a result, we can avoid ~200000 StringObject allocations in a single
test from the Speedometer 2 benchmark.

Co-Authored-By: Andreas Kling <andreas@ladybird.org>

Libraries/LibJS/Runtime/PropertyKey.h

@@ -105,6 +105,8 @@ public:
         return StringOrSymbol(as_symbol());
     }
 
+    bool operator==(PropertyKey const&) const = default;
+
 private:
     friend Traits<JS::PropertyKey>;