[Feature] allow emphasis to change z value

### What problem does this feature solve?

I want to bring emphasised series to front, so that it doesn't remain bellow other series.

I have a lot of lines overlapping, so this would allow me to bring in front the one user is interested in.

### What does the proposed API look like?

```js
const options = {
series: [{
type: 'line',
z: 2,
emphasis: {
z: 3,
},
}]
}
```