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
