LibJS: Add missing internal object string printing for debugging

When testing with JS_BYTECODE_DEBUG macro defined or using the All_Debug
profile, internal objects were not known in Value class to the function
Value::to_string_without_side_effects leading to a VERIFICATION FAILED
when running the test-js or test-web programs.

Internal objects are known in the Value class as cells, and do not have
a dedicated tag to identify them. Internal objects are detected using
is_cell function call, but only after all other types have been
checked as other types of non-internal objects can also be cells.

Both the String and Utf16String version of the function were updated.

Author: rcorsi

Libraries/LibJS/Runtime/Value.cpp

@@ -386,6 +386,8 @@ String Value::to_string_without_side_effects() const
     case EMPTY_TAG:
         return "<empty>"_string;
     default:
+        if (is_cell())
+            return String::formatted("[internal object {}]", as_cell().class_name()).release_value();
         VERIFY_NOT_REACHED();
     }
 }
@@ -417,6 +419,8 @@ Utf16String Value::to_utf16_string_without_side_effects() const
     case EMPTY_TAG:
         return "<empty>"_utf16;
     default:
+        if (is_cell())
+            return Utf16String::formatted("[internal object {}]", as_cell().class_name());
         VERIFY_NOT_REACHED();
     }
 }