GetOwnPropertyNames and GetPropertyNames should return Strings

[pinepain]

v8/v8@9009d21#diff-3dd7a0f46b1e5c1cc67921449e7180ac

Most of the users of these api methods manually ensure that the returned
values are Strings. With an additional flag we can easily ensure that already
in V8 and avoid needless api roundtrips.

Since: 6.6.231