[web] Add webMentionTypeaheadRegex tests

Summary:
This diff adds webMentionTypeaheadRegex regex unit tests to typeahead-utils.test.js.

Depends on D9056.

Test Plan: Run yarn test in web folder and check if all tests pass.

Reviewers: tomek, inka, rohan

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9057