[web][native] Rename usernamePrefix to textPrefix in TypeaheadMatchedStrings type

Summary:
This diff renames usernamePrefix to textPrefix in TypeaheadMatchedStrings type. Since we will show chats in mention typeahead tooltip, it is necessary to rename utilities to make code more generic.

Depends on D8853, D8849.

Test Plan: Check if TypeaheadTooltip works properly both on native and web.

Reviewers: tomek, inka, rohan

Reviewed By: tomek, rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8896