[lib/native/web] switch reactions data structure from map to plain object

Summary:
In dev mode the app was warning whenever a user pressed a message that "Non-serializble values were found in the navigation state". This was caused becasue reactions were stored in a map. To fix this, I switched the data structure from a map to a plain object

Linear Task: ENG-2851

Test Plan:
When a user taps on a message now the warning message about "non-serializable..." no longer shows up

Before:

After:

In addition tested on both native and web the following to make sure there were no regressions with reactions:

• reacting to a message (text/multimedia/robotext)
• unreacting to a message (text/multimedia/robotext)
• viewing who has liked a message
• sending multiple reactions to the same message

also flow

Reviewers: atul, tomek, ashoat, rohan

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6875