Add tachanun field to API output

[sdubois]

The purpose of the pull request is to expose the existing @hebcal/core tachanun calculation in @hebcal/rest-api

I thought it would be useful if this was available so people who make use of this service can also know when tachanun is said. Here are some example outputs:

Diaspora, January 2, 2026 (2026-01-02, Friday before Shabbat):

"tachanun": {
  "shacharit": true,
  "mincha": false,
  "allCongs": true
}

Diaspora, June 1, 2026 (2026-06-01, ordinary weekday):

"tachanun": {
  "shacharit": true,
  "mincha": true,
  "allCongs": true
}

Scope: add optional top-level tachanun field when options.tachanun is true and the response covers a single day
Test coverage: added formatter test for 2026-05-21

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[mjradwin]

Thank you for this PR! Will review and test and get back to you

[sdubois]

Thanks! I think there's also opportunity here to make the tachanun output more useful if you are open to it. Rather than just providing a boolean, it could also include a reason why tachanun is not said (eg, "chol hamoed"). For now I left this PR simple to just integrate with the existing functionality.

[mjradwin]

Do you want to attempt the change to @hebcal/core library (https://github.com/hebcal/hebcal-es6/blob/main/src/tachanun.ts) first to revise TachanunResult to add the tachanun reasons? To keep compatibility you'd want to keep the three existing boolean and then add additional string fields (or some enum if it makes more sense to do so)