KCL: Track Z axis of planes

KCL should track which way a plane is facing. If it only tracks a plane's
X and Y axes, it could identify two separate planes (overlapping
perfectly in space, but one plane's "up" is the other's down).

This is helpful for knowing which way to extrude on a given plane, or
which way to cut "into" a given solid.

rust/kcl-lib/std/types.kcl

@@ -210,6 +210,8 @@ export type fn
 /// ```
 ///
 /// Any object with appropriate `origin`, `xAxis`, and `yAxis` fields can be used as a plane.
+/// The plane's Z axis (i.e. which way is "up") will be the cross product X x Y. In other words,
+/// KCL planes follow the right-hand rule.
 @(impl = std_rust)
 export type Plane