[lib] Rename typeahead-utils.js to mention-utils.js

Summary:
This diff should be a no-op. I want to add some utility functions here that have to do with @-mentions, but not specifically the typeahead.

Depends on D6759

Test Plan: Flow

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6760