chore: rename StartReturningModel to Run

Added aliases for the old functions, but deprecated them.

examples/spinner/main.go

@@ -66,7 +66,7 @@ func (m model) View() string {
 
 func main() {
 	p := tea.NewProgram(initialModel())
-	if err := p.Start(); err != nil {
+	if _, err := p.Run(); err != nil {
 		fmt.Println(err)
 		os.Exit(1)
 	}