[lib] introduce stringForReactionList in reaction-utils

Summary:
introduce stringForReactionList in reaction-utils. This is a refactor where D5813 and D5814 were using the same logic in web and native, so we should put that logic into a helper function in lib. We then can call the function from web and native and get the same results

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Depends on D5812
Linear Task: ENG-2247

Test Plan:
Please to look at the screenshots to see that the reactions are still being rendered correctly

Native:

One reaction:

Multiple reactions:

Web:

One reaction:

Multiple reactions:

Reviewers: atul, tomek, rohan

Reviewed By: atul, rohan

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5878