feat: Add Juneteenth holiday to Time calendar

[mhargrove]

Add Juneteenth to the list of standard national holidays as per https://www.federalreserve.gov/aboutthefed/k8.htm

(this can be reverted if rickar/cal#93 is merged & released)

[adamdecaf]

I thought this was already handled by the library. I added it to the cal library before.

This was partially added, but fixed in https://github.com/rickar/cal/releases/tag/v2.1.5

[adamdecaf]

You can merge this and rebase #235 so we can merge that as well.

Releases are a git tag on the commit that updates the changelog. We copy that into the Github release