Add isLaunchRequest()

Today, we can easy to make canHandle method by isLaunchRequest.

before

const LaunchRequestHandler = {
  canHandle (handlerInput) {
    return canHandle(handlerInput, 'LaunchRequest')
  },

after

const LaunchRequestHandler = {
  canHandle (handlerInput) {
    return isLaunchRequest(handlerInput)
  },