UnitedKingdom support with England

[domino91]

No description provided.

[pich]

Great job🚀

[tbuczen]

+1

[pich]

@umulmrum Could you review this PR, merge and tag the new release?

[umulmrum]

Thank you very much for your PR and a thousand sorries for the long delay.

I created a new PR for the United Kingdom: #22
In this PR, I did a few things differently:

• Tried to dig through the many holiday rules and one-off holidays of the UK in the past few decades (this lib aims to be accurate for the time after WW2), and added most of them (omitted Scottish regional holidays as well as islands, just as you did); added loads of tests for different years because of that.
• Kept (Republic of) Ireland and UK separately as they are different countries.
• Didn't implement England explicitly, as it appears to have the same holidays as the UK as a whole. Note that you can reference England with both the GB and GB-ENG ISO codes even if the latter isn't defined explicitly - the lib falls back to the higher level GB automatically.

As your PR has been lying around for a year, I didn't want to bother you with rework wishes - in any case, thank you for the contribution and the inspiration! If you find mistakes in the new implementation, I'm happy to read issues and/or further PRs.