Add TypeScript definition

[sindresorhus]

Thanks for creating a TypeScript definition. 🙌 Can you follow this styleguide?

Some examples:

• https://github.com/sindresorhus/semver-regex/commit/039944bb5b3876913171b895488874b5c200e458
• https://github.com/sindresorhus/global-dirs/commit/0aae4d64887f77af94c4fb7ca12aa9f93c0a8291
• https://github.com/sindresorhus/md5-hex/commit/bbbfeff840f26973fbffc16d4863ae6d120079f3