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/StringOrSymbol.h

@@ -86,7 +86,7 @@ public:
     Value to_value(VM& vm) const
     {
         if (is_string())
-            return js_string(vm, as_string());
+            return PrimitiveString::create(vm, as_string());
         if (is_symbol())
             return const_cast<Symbol*>(as_symbol());
         return {};