LibJS: Move has_constructor() from NativeFunction to FunctionObject

At a later point this will indicate whether some FunctionObject "has a
[[Construct]] internal method" (separate from the current FunctionObject
call() / construct()), to help with a more spec-compliant implementation
of [[Call]] and [[Construct]].
This means that it is no longer relevant to just NativeFunction.

Userland/Libraries/LibJS/Runtime/FunctionObject.h

@@ -27,6 +27,8 @@ public:
 
     virtual bool is_strict_mode() const { return false; }
 
+    virtual bool has_constructor() const { return false; }
+
     // [[Realm]]
     virtual Realm* realm() const { return nullptr; }