Replace dependency emoji-regex with emoji-regex-xs

[slevithan]

This replaces emoji-regex (~13 KB uncompressed) with emoji-regex-xs (~0.3 KB), a drop-in replacement that shares its API and its 3,000+ tests, but reduces its file size by 97%. The difference is that it uses the version of Unicode supported natively by your environment, rather than hard coding code points from a specific version of Unicode.

I've also removed a TODO about replacing the regex with /\p{RGI_Emoji}/v when targeting Node.js 20, since RGI_Emoji is incomplete compared to real-world emoji use. The readme includes details, but essentially, a superset of RGI_Emoji is needed to match user expectations, since e.g. the iOS emoji keyboard overqualifies some emoji.