Flag sequences

[Enivex]

Unicode has two mechanisms for encoding regional flags

1. A pair of regional indicators (https://unicode.org/reports/tr51/#Flags). Only a subset of these pairs, determined by the CLDR, are valid. Valid pairs can never become invalid, only deprecated, so there's backwards compatibility.
2. Emoji tag sequences (https://unicode.org/reports/tr51/#valid-emoji-tag-sequences) for subregions. In principle this can be used for thousands of subregions, but due to lack of font support the only three marked as "recommended for general interchange" are the ones for the three countries making up the UK.

We should add support for these, for instance through flag.de, flag.gb, flag.un, flag.eu ((macro)regions) and flag.gb.eng (subregion). Only valid sequences should be accepted.

One question is whether support for other subregions than the UK ones should be added. I did find at least one font that supports more subregions (https://www.babelstone.co.uk/Fonts/Flags.html).