[lib] introduced notificationTexts function to reaction message spec

Summary:
introduced notificationTexts function to reaction message spec. Keeping the notification text generic for now with "user liked a message" and "user unlike a message", but will put up a follow up task to further improve the notification text because what I mention below is a bit out of the scope for the first version of message liking

1. Only send a notification if the user adds a like/reaction
2. Rescind a reaction notif and don't show anything new if the user removes a like/reaction
3. only the user who sent the target message gets a notification

Depends on D5690
Linear Task: ENG-2245

Test Plan:
Please watch the demo video to see the changes I made

Reviewers: tomek, rohan, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5870