Add TypeScript and ESM support

[mrmckeb]

This PR:

• Changes the underlying code to TypeScript.
• Exports strict types.
• Exports both ESM and CJS modules.

Resolves #159.

Todo:

• [x] Fix types by using export = instead of export default.
• [x] Modify the CJS file to use module.exports alongside exports.default.

[leerob]

Are we not running tests on each PR?