KCL: No 'let' or 'const' required when declaring vars (#4063)

Previously variable declaration required a keyword, e.g.

```kcl
let x = 4
const x = 4
var x = 4
```

These were all valid, and did the exact same thing. As of this PR, they're all still valid, but the KCL formatter will change them all to just:

```kcl
x = 4
```

which is the new preferred way to declare a constant. 

But the formatter will remove the var/let/const keywords.

Closes https://github.com/KittyCAD/modeling-app/issues/3985

src/wasm-lib/tests/executor/main.rs

@@ -2027,7 +2027,7 @@ const extrusion = extrude(10, sketch001)
 
 #[tokio::test(flavor = "multi_thread")]
 async fn kcl_test_angled_line_of_x_length_270() {
-    let code = r#"const sketch001 = startSketchOn('XZ')
+    let code = r#"sketch001 = startSketchOn('XZ')
   |> startProfileAt([0, 0], %)
   |> angledLineOfXLength({ angle: 90, length: 10 }, %, $edge1)
   |> angledLineOfXLength({ angle: -15, length: 20 }, %, $edge2)