chore: rename StartReturningModel to Run

Added aliases for the old functions, but deprecated them.

examples/progress-download/main.go

@@ -100,7 +100,7 @@ func main() {
 	// Start the download
 	go pw.Start()
 
-	if err := p.Start(); err != nil {
+	if _, err := p.Run(); err != nil {
 		fmt.Println("error running program:", err)
 		os.Exit(1)
 	}