Use typescript

[Zhomart]

• Moved comments to the index.ts.
• Genereates index.js/index.d.ts from the index.ts.
• Generated index.d.ts is similar to the original d.ts.
• Generated index.js is little different, but the tests pass.

Generated the package using npm pack, the final output was the same as the original package.

[sindresorhus]

I appreciate the pull request, but I'm not interested in moving to TypeScript for this package.