LibJS: Implement Object.hasOwn() :^)

This is currently a TC39 Stage 2 proposal, but let's go for it!

https://github.com/tc39/proposal-accessible-object-hasownproperty

I wrote the C++, @linusg found bugs and wrote the test.

Userland/Libraries/LibJS/Runtime/ObjectConstructor.h

@@ -42,6 +42,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(values);
     JS_DECLARE_NATIVE_FUNCTION(entries);
     JS_DECLARE_NATIVE_FUNCTION(create);
+    JS_DECLARE_NATIVE_FUNCTION(has_own);
 };
 
 }