[Annotation][Generator] change the @Pageflow grammar

The new grammar is as follows:

/**
 * @Pageflow({
 *     @StartPage({"input",
 *         @Transition("confirmation"),
 *     }),
 *     @Page({"confirmation",
 *         @Transition("success"),
 *         @Transition("input")
 *     }),
 *     @EndPage("success")
 * })
 */
[Conversation] rename Conversation::transitionTo() to transition()