[lib] Source onlyEmojiRegex from emoji-regex package

Summary:
The initial motivation for this diff was this security advisory I received from GitHub staff.

A secondary motivation is that this RegEx no longer works on all modern emoji. In particular, it's not matching "🧑🏽‍🎤👩🏼‍💼👩🏽‍🎤🧑‍🌾👨🏿‍🌾".

I think I initially copy-pasted this RegEx from the emoji-regex package, but I'm not 100% sure. Either way, this new approach will allow us to more easily stay updated as new emojis are added.

Test Plan: Open web client and test various emoji sequences, including the modern one above that didn't work before

Reviewers: KatPo, palys-swm

Reviewed By: KatPo, palys-swm

Subscribers: zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D456