fix: Change module exporting type (fix #159)

[junghwan-park]

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• It's submitted to right branch according to our branching model
• It's right issue type on title
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
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Description

In typescript, to import module by replacing entire exports object(module.exports = {}) must use import someModule = require('that-module');. ref.
module.exports = {} is not equivalent export default {}. Default export matches module.exports.default = {}.

case 1. Replacing exports

module.exports = ImageEditor;
// --------------------------------------

import ImageEditor = require('tui-image-editor');

case 2. Default export

module.exports.default = ImageEditor;
// or
export default ImageEditor;
// --------------------------------------

import ImageEditor from 'tui-image-editor';

Change module exporting type default to commonjs pattern.
ref: #159

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Thank you for your contribution to TOAST UI product. 🎉 😘 ✨

[dongsik-yoo]

수고하셨습니다.
docs 어딘가에 가이드를 명시해 주면 좋을 거 같아요.
관련 링크도 추가하면 좋겠습니다.
https://www.typescriptlang.org/docs/handbook/modules.html

[jinwoo-kim-nhn]

확인했습니다. 수고하셨습니다.

[junghwan-park]

@dongsik-yoo 네, 타입스크립트 모듈 import 설명을 README.md 하단에 추가했습니다!