Export RGI_Emoji as a separate property

[mathiasbynens]

https://unicode.org/reports/tr51/#def_rgi_set

ED-27. RGI emoji set — The set of all emoji (characters and sequences) covered by ED-20, ED-21, ED-22, ED-23, ED-24, and ED-25.

In other words:

RGI_Emoji =
    Basic_Emoji
  + Emoji_Keycap_Sequence
  + RGI_Emoji_Modifier_Sequence
  + RGI_Emoji_Flag_Sequence
  + RGI_Emoji_Tag_Sequence
  + RGI_Emoji_ZWJ_Sequence

All of those properties are enumerated in emoji-sequences.txt except for RGI_Emoji_ZWJ_Sequence which is in emoji-zwj-sequences.txt.

[mathiasbynens]

Doing this would simplify the emoji-regex build script, and make it more future-proof / less likely to require changes in the future.

[mathiasbynens]

This is released in @unicode/unicode-13.0.0@1.0.1. Here's the patch simplifying emoji-regex as a result: mathiasbynens/emoji-regex#71