chore: rename StartReturningModel to Run

Added aliases for the old functions, but deprecated them.

examples/http/main.go

@@ -26,7 +26,7 @@ func (e errMsg) Error() string { return e.error.Error() }
 
 func main() {
 	p := tea.NewProgram(model{})
-	if err := p.Start(); err != nil {
+	if _, err := p.Run(); err != nil {
 		log.Fatal(err)
 	}
 }