feature request - support an export entrypoint for messagingApi

[sacrosanctic]

It would be nice if I could import like this

import { MessagingApiClient } from '@line/bot-sdk/messagingApi ';

Currently, these are the import options

// ES modules or TypeScript
import { messagingApi } from '@line/bot-sdk';
const { MessagingApiClient } = messagingApi;
// OR
import * as line from '@line/bot-sdk';
const MessagingApiClient = line.messagingApi.MessagingApiClient;

// CommonJS
const MessagingApiClient = require('@line/bot-sdk').messagingApi.MessagingApiClient;