Should we allow to use strings as props as well? Which problems could this bring?

Right now we could set them using any string in the `foo['bar']` syntax (and call them in the same way), like `foo['0'] = 10px`, `width: foo['0']` etc, so we should either disallow the usage of strings that couldn't be used as idents for keys or we should allow to use strings in `{}` setters, so you could do `foo = { '0': 10px }`.