[lib] Introduce chat mention match and parse utilities

Summary:
Solution for ENG-4557.

This diff introduces decoding utilities which are going to be used in rules.react.js files both on web and native.

Depends on D8833.

Test Plan: Tested later in the stack.

Reviewers: tomek, inka, rohan

Reviewed By: inka, rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8834