Add Bulgarian holidays

[DanailPenev]

Summary

• This PR adds support for Bulgarian holidays, in line with other existing regions

Interesting difference

• The only interesting difference is that Bulgaria celebrates Orthodox Easter which can sometimes happen in early May
• Bulgaria also celebrates Labour Day on 1st May. When Labour Day is on the Saturday/Sunday right after Good Friday, the government moves Labour Day to Tuesday (instead of the Monday as per the usual rules)
• To accommodate for that, there is a special inline func that determines whether Labour Day should be observed on Monday or Tuesday

Tests

• Added tests for years 2018-2023.
• The government changed the observance rules in 2017 so it is expected that any dates before 2017 will not be accurate.

[DanailPenev]

@rickar please let me know if there is anything I've missed. 🙏

[rickar]

Looks good, thank you!