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/tests/var_ref_in_own_def/artifact_commands.snap

@@ -0,0 +1,32 @@
+---
+source: kcl-lib/src/simulation_tests.rs
+description: Artifact commands var_ref_in_own_def.kcl
+---
+[
+  {
+    "cmdId": "[uuid]",
+    "range": [],
+    "command": {
+      "type": "edge_lines_visible",
+      "hidden": false
+    }
+  },
+  {
+    "cmdId": "[uuid]",
+    "range": [],
+    "command": {
+      "type": "object_visible",
+      "object_id": "[uuid]",
+      "hidden": true
+    }
+  },
+  {
+    "cmdId": "[uuid]",
+    "range": [],
+    "command": {
+      "type": "object_visible",
+      "object_id": "[uuid]",
+      "hidden": true
+    }
+  }
+]

rust/kcl-lib/tests/var_ref_in_own_def/artifact_graph_flowchart.snap

@@ -0,0 +1,6 @@
+---
+source: kcl-lib/src/simulation_tests.rs
+description: Artifact graph flowchart var_ref_in_own_def.kcl
+extension: md
+snapshot_kind: binary
+---

rust/kcl-lib/tests/var_ref_in_own_def/artifact_graph_flowchart.snap.md

@@ -0,0 +1,3 @@
+```mermaid
+flowchart LR
+```

rust/kcl-lib/tests/var_ref_in_own_def/ast.snap

@@ -0,0 +1,75 @@
+---
+source: kcl-lib/src/simulation_tests.rs
+description: Result of parsing var_ref_in_own_def.kcl
+---
+{
+  "Ok": {
+    "body": [
+      {
+        "commentStart": 0,
+        "declaration": {
+          "commentStart": 0,
+          "end": 0,
+          "id": {
+            "commentStart": 0,
+            "end": 0,
+            "name": "x",
+            "start": 0,
+            "type": "Identifier"
+          },
+          "init": {
+            "callee": {
+              "abs_path": false,
+              "commentStart": 0,
+              "end": 0,
+              "name": {
+                "commentStart": 0,
+                "end": 0,
+                "name": "cos",
+                "start": 0,
+                "type": "Identifier"
+              },
+              "path": [],
+              "start": 0,
+              "type": "Name"
+            },
+            "commentStart": 0,
+            "end": 0,
+            "start": 0,
+            "type": "CallExpressionKw",
+            "type": "CallExpressionKw",
+            "unlabeled": {
+              "abs_path": false,
+              "commentStart": 0,
+              "end": 0,
+              "name": {
+                "commentStart": 0,
+                "end": 0,
+                "name": "x",
+                "start": 0,
+                "type": "Identifier"
+              },
+              "path": [],
+              "start": 0,
+              "type": "Name",
+              "type": "Name"
+            }
+          },
+          "start": 0,
+          "type": "VariableDeclarator"
+        },
+        "end": 0,
+        "kind": "const",
+        "preComments": [
+          "// This won't work, because `x` is being referenced in its own definition."
+        ],
+        "start": 0,
+        "type": "VariableDeclaration",
+        "type": "VariableDeclaration"
+      }
+    ],
+    "commentStart": 0,
+    "end": 0,
+    "start": 0
+  }
+}

rust/kcl-lib/tests/var_ref_in_own_def/execution_error.snap

@@ -0,0 +1,13 @@
+---
+source: kcl-lib/src/simulation_tests.rs
+description: Error from executing var_ref_in_own_def.kcl
+---
+KCL UndefinedValue error
+
+  × undefined value: `x` is not defined
+   ╭─[2:9]
+ 1 │ // This won't work, because `x` is being referenced in its own definition.
+ 2 │ x = cos(x)
+   ·         ┬
+   ·         ╰── tests/var_ref_in_own_def/input.kcl
+   ╰────

rust/kcl-lib/tests/var_ref_in_own_def/input.kcl

@@ -0,0 +1,2 @@
+// This won't work, because `x` is being referenced in its own definition.
+x = cos(x)

rust/kcl-lib/tests/var_ref_in_own_def/ops.snap

@@ -0,0 +1,5 @@
+---
+source: kcl-lib/src/simulation_tests.rs
+description: Operations executed var_ref_in_own_def.kcl
+---
+[]

rust/kcl-lib/tests/var_ref_in_own_def/unparsed.snap

@@ -0,0 +1,6 @@
+---
+source: kcl-lib/src/simulation_tests.rs
+description: Result of unparsing var_ref_in_own_def.kcl
+---
+// This won't work, because `x` is being referenced in its own definition.
+x = cos(x)