Implement relative-layout-mode `<constraint />` between groups

We want to be able to do something like:

```tsx
<constraint pcb xDist="20mm" left=".group1" right=".group2" centerX={0} />
```

To set the distance between two groups. `centerX` is optional but in this case allows you to easily compute the position of both groups on the x-axis

- [ ] Introduce any necessary props
- [ ] Create test
- [ ] Implement constraint


/bounty $200