Common specification for the cron pattern

This is not an issue connected to this library per se, but while implementing Cron-parsers for JS/TS (croner) and Rust (croner-rust), it became apparent that there is no common specification for the cron format, especially the extensions like `W` `L` and `#`.

To tackle that, I've started work on an open specification - [OCPS (Open Cron Pattern Specification)](https://github.com/open-source-cron/ocps), and are reching out for more cron-parser library authors for collaboration.

Anyone here who would like to join as a collaborator?