Provide commonjs version

[just-boris]

Thank you for the wonderful library, it is a really handy utility!

However, currently it is hard to integrate this into an project, because it ships only es-modules version. In particular, it causes problems with Jest, that does not have es-modules support. (esm also does not work).

Currently I have to work it around via running Babel for node_modules (recommended here standard-things/esm#706 (comment)), which has its own set of problems.

Most of the npm-modules, still ship commonjs version, so mnth is one of the few causing this issue.

[deepsweet]

I've removed ESM loader from all my libs recently and just forgot about mnth. Will be definitely fixed.

[deepsweet]

Landed with v0.1.2

[just-boris]

Thank you!