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/simulation_tests.rs

@@ -3483,3 +3483,24 @@ mod spheres {
         super::execute(TEST_NAME, true).await
     }
 }
+mod var_ref_in_own_def {
+    const TEST_NAME: &str = "var_ref_in_own_def";
+
+    /// Test parsing KCL.
+    #[test]
+    fn parse() {
+        super::parse(TEST_NAME)
+    }
+
+    /// Test that parsing and unparsing KCL produces the original KCL input.
+    #[tokio::test(flavor = "multi_thread")]
+    async fn unparse() {
+        super::unparse(TEST_NAME).await
+    }
+
+    /// Test that KCL is executed correctly.
+    #[tokio::test(flavor = "multi_thread")]
+    async fn kcl_test_execute() {
+        super::execute(TEST_NAME, true).await
+    }
+}