LibJS: Use allocate_without_global_object for primitive cell types

More steps towards multiple global object support. Primitive cells
like strings, bigints, etc, don't actually have any connection to
the global object. Use the explicit API to clarify this.

Libraries/LibJS/Runtime/PrimitiveString.cpp

@@ -41,7 +41,7 @@ PrimitiveString::~PrimitiveString()
 
 PrimitiveString* js_string(Heap& heap, String string)
 {
-    return heap.allocate<PrimitiveString>(heap.interpreter().global_object(), move(string));
+    return heap.allocate_without_global_object<PrimitiveString>(move(string));
 }
 
 PrimitiveString* js_string(Interpreter& interpreter, String string)