Add country Greenland (GL)

The following holidays need to be verified/completed:

|Date | Flags | Name|
| --- | --- | --- |
| 01-01 | [NF] | New Year's Day|
| 01-06 | [NF] | Epiphany|
| 06-21 | [NF] | Ullortuneq|
| 12-24 | [NRF] | Christmas Eve|
| 12-25 | [NRF] | Christmas Day|
| 12-26 | [NRF] | St Stephen's Day|
| 12-31 | [NF] | New Year's Eve|
| 3 days before Easter | [NRV] | Maundy Thursday|
| 2 days before Easter | [NRV] | Good Friday|
| 1 day after Easter | [NRV] | Easter Monday|
| | [NRV] | General Prayer Day|
| 39 days after Easter | [NRV] | Ascension|
| 50 days after Easter | [NRV] | Whit Monday|

Checklist:
* What type is the easter date?
- [ ] orthodox
- [x] 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

Start by reviewing the repository's existing country holiday definitions and verifying Greenland's listed holidays against authoritative legal sources. Confirm the western Easter calculations, the missing General Prayer Day date, holiday names, and fixed dates; done means Greenland is added with complete, sourced, correct holiday data.

Written by the indexing model from the issue text.