Adds an `arrayReduce` function to KCL stdlib. Right now, it can only reduce SketchGroup values because my implementation of higher-order KCL functions sucks. But we will generalize it in the future to be able to reduce any type.
This simplifies sketching polygons, e.g.
```
fn decagon = (radius) => {
let step = (1/10) * tau()
let sketch = startSketchAt([
(cos(0) * radius),
(sin(0) * radius),
])
return arrayReduce([1..10], sketch, (i, sg) => {
let x = cos(step * i) * radius
let y = sin(step * i) * radius
return lineTo([x, y], sg)
})
}
```
Part of #3842
