MaterialTextField choice add SelectedItem

I used MaterialTextField for used like picker in my app , Please add Selecteditem property in it so We can directly get selected Item without calling SelectedItemChanged event.

Locate the MaterialTextField implementation and inspect how SelectedItemChanged is declared and raised. Compare its existing property and binding patterns, then verify that a direct SelectedItem value is available without subscribing to the event.

Written by the indexing model from the issue text.