feat: add business days utils

[khalidwilliams]

[Hack Day] Add a series of functions for working with business days, previously kept in the homebound-team/date-fns repo. The aim is to eventually remove this fork / its patches from other Homebound repos.

[zgavin]

lazy ask, are these just copy/pasted from the previous patch?

[khalidwilliams]

@zgavin yep, with the tests moved over from mocha to jest

[homebound-team-bot]

🎉 This PR is included in version 1.7.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[stephenh]

@khalidwilliams / @blambillotte I suppose this is somewhat comical at this point (where we put these functions :-)), but personally I would not have added them to activesupport, as I generally see activesupport as "something anyone (non-HB-even) could drop into their project and get ergonomic DX for javascript's core arrays/etc data structures".

These are basically "our custom date-fns" and imo should be in a standalone @homebound/date-fn-utils or @homebound/business-days repo.

For awhile, we had ~good infra to setup in ts libraries + repositories, i.e. see the rtl-utils & friends repos that are ~handful-of-line repos that have semver auto-publish to npm/CI/etc. setup. It's not too much work.