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/ECMAScriptFunctionObject.h

@@ -112,6 +112,8 @@ private:
     ThrowCompletionOr<void> function_declaration_instantiation();
 
     DeprecatedFlyString m_name;
+    GCPtr<PrimitiveString> m_name_string;
+
     GCPtr<Bytecode::Executable> m_bytecode_executable;
     Vector<NonnullGCPtr<Bytecode::Executable>> m_default_parameter_bytecode_executables;
     i32 m_function_length { 0 };