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/execution/fn_call.rs

@@ -318,10 +318,13 @@ impl Node<CallExpressionKw> {
                     if let KclValue::Function { meta, .. } = func {
                         source_ranges = meta.iter().map(|m| m.source_range).collect();
                     };
-                    KclError::new_undefined_value(KclErrorDetails::new(
-                        format!("Result of user-defined function {} is undefined", fn_name),
-                        source_ranges,
-                    ))
+                    KclError::new_undefined_value(
+                        KclErrorDetails::new(
+                            format!("Result of user-defined function {} is undefined", fn_name),
+                            source_ranges,
+                        ),
+                        None,
+                    )
                 })?;
 
                 Ok(result)