[lib/web/native] Convert getMentionTypeaheadUserSuggestions into a hook

Summary:
This is some feedback provided in https://phab.comm.dev/D10387#inline-63506. This diff converts getMentionTypeaheadUserSuggestions into a hook that is consumed in both web and native.

Addressess ENG-6278

Depends on D10407

Test Plan: Made sure the @ mentioning for users and chats still looked fine, nothing seemed slow or out of place. Also opened up the Chrome console to make sure no React warnings were appearing.

Reviewers: atul, ginsu, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10432