Ver4.0.0 Updates

Updates

Base Source TypeScript ✨

Changed base source from JavaScript to TypeScript.

5198a1e

source

src/countdown.js => src/countdown.ts

tslint.json => @typescript-eslint

typescript linter changed from tslint.json to @typescript-eslint

conpile target es5 => es6

https://github.com/yamanoku/birthday-countdown-js/blob/3c109172b8f171b7ba6e9824431b072d549fa91d/tsconfig.json#L4