LibJS: Make ExecutionContext::function_name a GCPtr<PrimitiveString>

This required setting things up so that all function objects can plop
a PrimitiveString there instead of an AK string.

This is a step towards making ExecutionContext easier to allocate.

Userland/Libraries/LibJS/Runtime/Error.cpp

@@ -85,7 +85,7 @@ void Error::populate_stack()
         if (element.source_range.has_value())
             range = element.source_range.value();
         TracebackFrame frame {
-            .function_name = context->function_name,
+            .function_name = context->function_name ? context->function_name->deprecated_string() : "",
             .source_range_storage = range,
         };