gridlines z(-order/index) callback function for (major/minor) layout

### Feature Proposal

With v3.0, we have now a number of styling options for major/minor gridlines by callback function like:
` gridLines: {
color: function(context) {
return context.tick.major ? (context.tick.value ? 'grey' : 'black') : 'lightgrey';
},
`
However, these individual styling options can create a problem with z-index in the sense for instance that major gridlines should be drawn on top of minor gridlines.

It would be nice if 'z' worked on individual gridlines and had a callback function option:
` z: function(context) {
return context.tick.major ? (context.tick.value ? -200 : -100) : -300;
},
`

### Feature Use Case

Here is a visual example of a z-index problem with major/minor gridline raster: https://i.imgur.com/onn6lSe.png

## Possible Implementation