[lib] Introduce chat mention utilities for plain text

Summary:
This diff introduces new chat mention utilities which will be needed when handling notification text.

Depends on D8945, D8910.

Test Plan: Tested later in the stack.

Reviewers: tomek, inka, rohan

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9005