[lib] Remove negative lookbehind assertions from chat mention regex

Summary: This diff resolves ENG-5150. This issue was caused by negative lookbehind assertions in chat mention regex which are not supported in Safari version lower than 16.4.

Test Plan:

1. Tested if new chat mention regex works exactly like version with negative lookbehind assertions on web, iOS, Android.
2. Checked if there are no errors in lib tests.
3. Tested if error still occurs on Safari versions < 16.4 using BrowserStack.

Reviewers: tomek, ashoat, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D9344