KCL: Getter for axes of planes (#7662)

## Goal

Currently, there's no way in KCL to get fields of a plane, e.g. the underlying X axis, Y axis or origin.

This would be useful for geometry calculations in KCL. It would help KCL users write transformations between planes for rotating geometry.

For example, this enables

```kcl
export fn crossProduct(@vectors) {
  a = vectors[0]
  b = vectors[1]
  x = a[1] * b[2] - (a[2] * b[1])
  y = a[2] * b[0] - (a[0] * b[2])
  z = a[0] * b[1] - (a[1] * b[0])
  return [x, y, z]
}

export fn normalOf(@plane) {
  return crossProduct([plane.xAxis, plane.yAxis])
}
```

## Implementation

My goal was just to enable a simple getter for planes, like `myPlane.xAxis` and yAxis and origins. That's nearly what happened, except I discovered that there's two ways to represent a plane: either `KclValue::Plane` or `KclValue::Object` with the right fields.

No matter which format your plane is represented as, it should behave consistently when you get its properties. Those properties should be returned as `[number; 3]` because that is how KCL represents points.

Unfortunately we actually require planes-as-objects to be defined with axes like `myPlane = { xAxis = { x = 1, y = 0, z = 0 }, ...}`, but that's a mistake in my opinion. So if you do use that representation of a plane, it should still return a [number; 3]. This required some futzing around so that we let you access object fields .x and .y as [0] and [1], which is weird, but whatever, I think it's good.

This PR is tested via planestuff.kcl which has a Rust unit test.

Part of the hole efforts, see https://github.com/KittyCAD/modeling-app/discussions/7543

rust/kcl-lib/tests/inputs/planestuff.kcl

@@ -0,0 +1,60 @@
+// There are 3 ways to define a plane in KCL, according to https://zoo.dev/docs/kcl-std/types/std-types-Plane
+//  - A default plane
+//  - Modifying a default plane e.g. via offsetPlane
+//  - Defining your own struct
+// This file tests they all work equivalently.
+
+// Define a plane using struct representation.
+myPlane = {
+    origin = { x = 0, y = 0, z = 0 },
+    xAxis = { x = 1, y = 0, z = 0 },
+    yAxis = { x = 0, y = 1, z = 0 },
+}
+
+// Prove we can get its axes and origin.
+ax = myPlane.xAxis
+assert(ax[0], isEqualTo = 1)
+assert(ax[1], isEqualTo = 0)
+assert(ax[2], isEqualTo = 0)
+ay = myPlane.yAxis
+assert(ay[0], isEqualTo = 0)
+assert(ay[1], isEqualTo = 1)
+assert(ay[2], isEqualTo = 0)
+aorigin = myPlane.origin
+assert(aorigin[0], isEqualTo = 0)
+assert(aorigin[1], isEqualTo = 0)
+assert(aorigin[2], isEqualTo = 0)
+
+// Define a plane using standard planes.
+myOtherPlane = XY
+
+// Prove we can get its axes and origin.
+axOther = myOtherPlane.xAxis
+assert(axOther[0], isEqualTo = 1)
+assert(axOther[1], isEqualTo = 0)
+assert(axOther[2], isEqualTo = 0)
+ayOther = myOtherPlane.yAxis
+assert(ayOther[0], isEqualTo = 0)
+assert(ayOther[1], isEqualTo = 1)
+assert(ayOther[2], isEqualTo = 0)
+aoriginOther = myOtherPlane.origin
+assert(aoriginOther[0], isEqualTo = 0)
+assert(aoriginOther[1], isEqualTo = 0)
+assert(aoriginOther[2], isEqualTo = 0)
+
+// Define a plane using a plane-modifying function like offsetPlane.
+myAlternatePlane = offsetPlane(XY, offset = 0)
+
+// Prove we can get its axes and origin.
+axAlternate = myAlternatePlane.xAxis
+assert(axAlternate[0], isEqualTo = 1)
+assert(axAlternate[1], isEqualTo = 0)
+assert(axAlternate[2], isEqualTo = 0)
+ayAlternate = myAlternatePlane.yAxis
+assert(ayAlternate[0], isEqualTo = 0)
+assert(ayAlternate[1], isEqualTo = 1)
+assert(ayAlternate[2], isEqualTo = 0)
+aoriginAlternate = myAlternatePlane.origin
+assert(aoriginAlternate[0], isEqualTo = 0)
+assert(aoriginAlternate[1], isEqualTo = 0)
+assert(aoriginAlternate[2], isEqualTo = 0)