KCL stdlib reduce function (#3881)

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

docs/kcl/index.md

@@ -19,6 +19,7 @@ layout: manual
 * [`angledLineToX`](kcl/angledLineToX)
 * [`angledLineToY`](kcl/angledLineToY)
 * [`arc`](kcl/arc)
+* [`arrayReduce`](kcl/arrayReduce)
 * [`asin`](kcl/asin)
 * [`assert`](kcl/assert)
 * [`assertEqual`](kcl/assertEqual)