LibJS: Replace standalone js_string() with PrimitiveString::create()

Note that js_rope_string() has been folded into this, the old name was
misleading - it would not always create a rope string, only if both
sides are not empty strings. Use a three-argument create() overload
instead.

Userland/Libraries/LibJS/Runtime/DateConstructor.cpp

@@ -202,7 +202,7 @@ ThrowCompletionOr<Value> DateConstructor::call()
     auto now = AK::Time::now_realtime().to_milliseconds();
 
     //     b. Return ToDateString(now).
-    return js_string(vm(), to_date_string(now));
+    return PrimitiveString::create(vm(), to_date_string(now));
 }
 
 // 21.4.2.1 Date ( ...values ), https://tc39.es/ecma262/#sec-date