Is your feature request related to a problem? Please describe.
When integrating Huh into a larger bubble tea application it's sometimes useful to change behavior based on the selected field in a form. Some examples:

1. We're building a form with a lot of fields and have decided to split the form up and put each sub form into a tab component. It would be helpful to know when a user has focused the last field on a form so that our tab component could respond to the next tab key press to move the user into the next tab with the next section of the form.
2. We may want to show more information about a field in a side panel next to the form, such as documentation or other helpful information that's too large to fit into the form itself.

Describe the solution you'd like
I'd like to have a GetFocusedField function on the form to return the current field (either the entire field struct, or at least the key), and it's relative position in the form and group.

Describe alternatives you've considered

Additional context
Here's an example of the form in tab component:

Loving the library, it makes creating rich TUI forms so easy!