LibJS: Decouple new_function_environment() from FunctionObject

Now that only ECMAScriptFunctionObject uses this, we can remove the
FunctionObject::new_function_environment() pure virtual method and just
implement it as a standalone AO with an ECMAScriptFunctionObject
parameter, next to the other NewFooEnvironment AOs.

Userland/Libraries/LibJS/Runtime/ECMAScriptFunctionObject.h

@@ -79,7 +79,6 @@ protected:
 
 private:
     virtual bool is_ecmascript_function_object() const override { return true; }
-    virtual FunctionEnvironment* new_function_environment(Object* new_target) override;
     virtual void visit_edges(Visitor&) override;
 
     void prepare_for_ordinary_call(ExecutionContext& callee_context, Object* new_target);