KCL: Use named fields for KclError (#7321)

We've changed the unnamed field of `KclError` variants to a named called `details`.

To clarify: previously KCL errors looked like this:

```rust
pub enum KclError {
    Lexical(KclErrorDetails),
    Syntax(KclErrorDetails),
```

Now they look like this:

```rust
pub enum KclError {
    Lexical { details: KclErrorDetails },
    Syntax { details: KclErrorDetails },
}
```

This lets us more easily add fields to the errors. For example, in the UndefinedValue case, adding a field for what the undefined name was. This PR refactors the code to make my PR in https://github.com/KittyCAD/modeling-app/pull/7309 much easier.

Pure refactor, should not change any behaviour.

src/lib/rustContext.ts

@@ -141,7 +141,7 @@ export default class RustContext {
       )
     } catch (e: any) {
       const parsed: RustKclError = JSON.parse(e.toString())
-      toast.error(parsed.msg, { id: toastId })
+      toast.error(parsed.details.msg, { id: toastId })
       return
     }
   }