Brexit and VAT refactor

[kloon]

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Changes proposed in this Pull Request:

This PR does a couple of things. First it refactors the VAT functionality currently built into the WC_Countries::get_european_union_countries method by deprecating it and introducing a new method WC_Countries::get_vat_countries and updating all calls to the get_european_union_countries method for VAT purposes with the new method.

Secondly, it removes GB from the get_european_union_countries method and then add them to the get_vat_countries method since GB still uses VAT.

It is important to note that this PR is still in progress and should not be merged before Brexit officially happens. From what I could see there should be no effect on having GB remain as part of the get_european_union_countries in core since we only use that for determining to use VAT or TAX, but it could have an effect on plugins that relies on the method for specific functionality relating to EU countries, for that there is a filter currently in place that can be utilised to remove GB should it be needed in these plugins.

Closes #24666

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Changelog entry

Dev - Introduce WC_Countries::get_vat_countries for returning a list of countries that uses VAT and refactor WC_Countries::get_european_union_countries with backward compatibility and deprecation to remove the VAT functionality from there.
Dev - Brexit, remove GB from WC_Countries::get_european_union_countries

[rodrigoprimo]

Looks good to me overall, but there is one problem that needs to be addressed that I mentioned in an inline comment.

Besides that, I wonder if we should keep GB in the list of european countries in this PR. This way, we can merge it for WC 3.9 as just the refactor is already an improvement and should make it easier for those that need to adjust their store using the filters provided here when Brexit happens and before we release a new WooCommerce version. Then we can address removing GB on a separate PR when the time comes. What do you think?

[claudiosanches]

I'm fixing the conflicts.

[claudiosanches]

I have updated this PR to use WC_Countries::countries_using_vat() introduced in #24999

[peterfabian]

I think we can improve this one a bit, please see below.

Fixed now, I believe.