Add KCL constants for sweep relativeTo strings (#6974)

* Add KCL constants for sweep relativeTo strings

Before:

```
sweep(pill, path = sweepPath, relativeTo = "trajectoryCurve")
```

After:

```
sweep(pill, path = sweepPath, relativeTo = sweep::TRAJECTORY)
```

* Update docs

rust/kcl-lib/src/modules.rs

@@ -96,6 +96,7 @@ pub(crate) fn read_std(mod_name: &str) -> Option<&'static str> {
         "solid" => Some(include_str!("../std/solid.kcl")),
         "units" => Some(include_str!("../std/units.kcl")),
         "array" => Some(include_str!("../std/array.kcl")),
+        "sweep" => Some(include_str!("../std/sweep.kcl")),
         "transform" => Some(include_str!("../std/transform.kcl")),
         _ => None,
     }