[Feature] Calendar coordinates: Customize gap between months

### What problem does this feature solve?

Allow customizing the gap between months (similar to [cal-heatmap](https://cal-heatmap.com/docs/options/domain#gutter))

This would allow you to achieve similar layout to cal-heatmap:

### What does the proposed API look like?

```
option = {
calendar: {
splitGap: number
}
}
```