KCL: Fix 'cryptic' error when referencing a variable in its own declaration (#7325)

Previously, `x = cos(x)` would just say "`x` is undefined". Now it says that `x` cannot be referenced in its own definition, try using a different variable instead.

To do this, I've added a new `Option<String>` field to the mod-local executor context, tracking the current variable declaration. This means cloning some strings, implying a small performance hit. I think it's fine, for the better diagnostics.

In the future we could refactor this to use a &str or store variable labels in stack-allocated strings like docs.rs/compact_str or something.

Closes https://github.com/KittyCAD/modeling-app/issues/6072

rust/kcl-lib/src/std/csg.rs

@@ -23,7 +23,7 @@ pub async fn union(exec_state: &mut ExecState, args: Args) -> Result<KclValue, K
     let tolerance: Option<TyF64> = args.get_kw_arg_opt_typed("tolerance", &RuntimeType::length(), exec_state)?;
 
     if solids.len() < 2 {
-        return Err(KclError::new_undefined_value(KclErrorDetails::new(
+        return Err(KclError::new_semantic(KclErrorDetails::new(
             "At least two solids are required for a union operation.".to_string(),
             vec![args.source_range],
         )));
@@ -88,7 +88,7 @@ pub async fn intersect(exec_state: &mut ExecState, args: Args) -> Result<KclValu
     let tolerance: Option<TyF64> = args.get_kw_arg_opt_typed("tolerance", &RuntimeType::length(), exec_state)?;
 
     if solids.len() < 2 {
-        return Err(KclError::new_undefined_value(KclErrorDetails::new(
+        return Err(KclError::new_semantic(KclErrorDetails::new(
             "At least two solids are required for an intersect operation.".to_string(),
             vec![args.source_range],
         )));