Pseudo-elements don't work with pseudoclasses

We enforce top-level pseudo-elements, but given something like so:

```js
const styles = stylex.create({
    foo: {
    "::before": {
      content: '"✓"',
      color: {
        default: 'yellow',
        ':hover': 'red'
      },
   }
})
```

The outputted CSS is as follows, which is invalid, as pseudo-elements must after at the end of the selector.
```css
.x1qdmp1q::before:hover{color:red}
```