[lib] Use RegExp.source instead of RegExp.toString() in emojis.js

Summary:
RegExp.toString() returns a string that includes / at the start and /g at the end, which forces us to parse the resultant string. We do this currently with another RegExp.

However, RegExp.source doesn't include those things. By using this property we can skip the parsing step.

Depends on D8145

Test Plan: Unit tests and comparing the generated RegExp object before and after the change to make sure they match (in Chrome debugger)

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8146