How to use the bottender.ViberHandler function in bottender

const { ViberBot, ViberHandler } = require('bottender');
const { createServer } = require('bottender/express');

const config = require('./bottender.config').viber;

const bot = new ViberBot({
  accessToken: config.accessToken,
});

const handler = new ViberHandler()
  .onDelivered(() => {
    console.log('delivered');
  })
  .onSeen(() => {
    console.log('seen');
  })
  .onFailed(() => {
    console.log('failed');
  })
  .onText(/yo/i, async context => {
    await context.sendText('Hi there!');
  })
  .onEvent(async context => {
    await context.sendText("I don't know what you say.");
  })
  .onError(async context => {