[web] introduce ReactionPill component

Summary:
As I was working on implementing the reaction tooltip I was having to put a ton of reaction specific code into InlineEngagement. This was getting messy really quick so I created and introduced this ReactionPill component to keep the logic organized

Depends on D8810

Test Plan: flow and no regressions with the ReactionPill (The pill looks the same and I can still react/unreact by clicking on the pill)

Reviewers: atul, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8819