Add `regional_indicator_*` / big letter emojis

[super-cooper]

emoji currently supports these emojis combined together to form flag codes, such as:

':Afghanistan:' = '🇦🇫'

The 🇦🇫 emoji is actually a combination of two emojis: 🇦 and 🇫. There is currently no way to emojize/demojize to these single character "regional indicator emojis."

In Discord and the GNOME Characters app, these are known as "Regional Indicators," but they are not an official part of the unicode spec (besides as pieces of flags).

If they were to be included, an example of their mapping could be:

':regional_indicator_f:' = '🇫'

This is the naming convention Discord chooses for these symbols, but it may not be the best choice.