KCL: If-else expressions (#4022)

Closes https://github.com/KittyCAD/modeling-app/issues/3677

You can review each commit separately, they're neat commits with logical purpose in each.

Future enhancements:

- https://github.com/KittyCAD/modeling-app/issues/4015
- https://github.com/KittyCAD/modeling-app/issues/4020
- Right now the parser errors are not very good, especially if you forget to put an expression in the end of an if/else block

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

@@ -90,4 +90,9 @@ gen_test_fail!(
     "semantic: cannot use % outside a pipe expression"
 );
 gen_test!(sketch_in_object);
+gen_test!(if_else);
+// gen_test_fail!(
+//     if_else_no_expr,
+//     "syntax: blocks inside an if/else expression must end in an expression"
+// );
 gen_test!(add_lots);