Support icon classes

### Is your feature request related to a problem? Please describe.

The add, remove, move-up and move-down buttons are rather ugly with just a textual `+` and `-` sign.

### Describe the solution you'd like

I would like to be able configure the Vanilla renderer to use icon classes so I can use the icons from [Bootstap Icons](https://icons.getbootstrap.com/) or [Font Awesome](https://fontawesome.com/).

### Describe alternatives you've considered

Fork the Vanilla renderer.

### Framework

Vue 3

### RendererSet

Vanilla

### Additional context

_No response_