Consider warning against data keys starting with `-ms-`

[mgol]

In 4.0, data no longer treats -ms ins a special way when camelcasing. In 3.x this was happening because the camelCasing logic was shared with the CSS module which needed it.

Consider warning against such usage. This should be pretty rare and there's no obvious proper way to fix it, so a warning should probably be enough.

If we decide to land it, it should go into both 3.x & 4.x lines.