Add support for exponents without caret symbol

Currently writing `3²` results in an error, but I think it would be a nice-to-have feature to have Kalkki calculate it correctly. At least SpeedCrunch supports this functionality and it converts the expression to `3^2` when the user presses enter.

<img width="708" alt="Image" src="https://github.com/user-attachments/assets/fe59c266-2890-4ec7-918b-a54f1cda1e67" />