LibJS: Implement the PrepareForOrdinaryCall abstract operation

This is used by VM::call_internal() and VM::construct() which roughly
map to function objects' [[Call]] and [[Construct]] slots in the spec.

Reorganizing this code revealed something weird: NativeFunction gets
its strictness by checking VM::in_strict_mode(). In other words,
it inherits the strict flag from the caller context. This is quite
weird, but many test-js tests rely on it, so let's preserve it until
we can think of something nicer.

Userland/Libraries/LibJS/Runtime/VM.h

@@ -260,6 +260,7 @@ private:
     VM();
 
     [[nodiscard]] Value call_internal(FunctionObject&, Value this_value, Optional<MarkedValueList> arguments);
+    void prepare_for_ordinary_call(FunctionObject&, ExecutionContext& callee_context, Value new_target);
 
     Exception* m_exception { nullptr };