[lib] Extract notifTextsForSidebarCreation

Summary:
In a later diff, we'll introduce a new callsite for this from text-message-spec.js.

Depends on D6936

Test Plan:
Flow, and tested in combination with the next diffs to confirm that getting @-tagged in a message that triggers the creation of a sidebar will update the SIDEBAR_CREATION notif
to be suffixed with "and tagged you"

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6937