Add country Latvia (LV)

The following list of holidays needs to be verified/completed:
|Date | Flags | Name|
| --- | --- | --- |
| 01-01 | [NF] | Jaunais Gads |
| 05-01 | [NF] | Darba svētki |
| 05-05 | [NF] | Latvijas Republikas Neatkarības atjaunošanas dienu |
| 06-24 | [NF] | Jāņi |
| 11-18 | [NF] | Latvijas Republikas proklamēšanas diena |
| 12-24 | [NRF] | Ziemassvētku vakars |
| 12-25 | [NRF] | Ziemassvētki |
| 12-26 | [NRF] | 2. Ziemassvētki |
| 12-31 | [NF] | Vecgada vakars |
| | [NRV] | Lielā Piektdiena |
| | [NRV] | Lieldienas |
| | [NRV] | 2. Lieldienas |

Checklist:
* What type is the easter date?
- [ ] 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 source file or test is named in the issue. Start by locating the existing country holiday definitions and their tests, then verify Latvia's fixed and movable holidays, Easter type, legal sources, names, and dates; done means the Latvia entry is complete and the checklist is satisfied.

Written by the indexing model from the issue text.