Add country Belarus (BY)

The following holidays need to be verified:

|Date | Flags | Name|
| --- | --- | --- |
| 01-01 | [NF] | Новы Год |
| 01-07 | [NRF] | Раство Хрыстова(праваслаўнае) |
| 08-03 | [NF] | Міжнародны жаночы дзень |
| 05-01 | [NF] | Свята працы |
| 05-09 | [NF] | Дзень Перамогі |
| 07-03 | [NF] | Дзень незалежнаці |
| 11-07 | [NF] | Дзень Кастрычніцкай рэвалюцыі |
| 12-25 | [NRF] | Раство Хрыстова(у заходніх хрысціян) |
| Easter | [NRV] | Пасха(праваслаўная) |
| 15 days after Easter | [NRV] | Радуніца |

Checklist:
* What type is the easter date?
- [x] orthodox
- [ ] western
- [ ] All holidays (i.e. days "which are defined by law on which business or work are suspended or reduced") are listed
- [ ] Legal sources which define the holidays are provided
- [ ] The spelling of the holiday names is correct
- [ ] The given dates (independent of year) of the holidays are correct

No files, tests, or entry points are named; first locate the existing country holiday definitions and their tests, then review how Orthodox Easter and relative dates are represented. Done means adding Belarus (BY) with the listed holidays, verifying names and fixed dates, documenting legal sources, and confirming the Easter date type.

Written by the indexing model from the issue text.