[lib] Utils for typeahead mentions

Summary:
Preparation for typeahead user suggestions feature.
Utilities: - function getTypeaheadUserSuggestions() used to filter out users from current thread based on already typed prefix.

Also added stringForUserExplicit(), because when we use @-ing and we @ ourselves, we want to use our name, not "you",
e.g. I'd type: @przemek, instead od @you, which feels weird.

Test Plan:
This is not called anywhere yet.
I runned web app and it still works the same.
Checked visibility of functions.

Reviewers: tomek, kamil, atul

Reviewed By: tomek

Subscribers: ashoat, abosh, atul, kamil, tomek

Differential Revision: https://phab.comm.dev/D5532