[lib] Return style of text from getMessagePreview

Summary:
I noticed that native/chat/message-preview.react.js wasn't using getMessagePreview, even though much of the logic was the same.

I wanted to unify the codepaths, but there is was one thing that native needed that wasn't being returned by getMessagePreview: what style to render each of the parts as.

I also noticed that web was rendering styles slightly different that native, which will be fixed in the next diff. This diff doesn't actually use the newly-returned style field... it just updates the code to return it.

Test Plan: Tested in combo with following diffs

Reviewers: inka, ginsu, atul, tomek

Reviewed By: inka

Differential Revision: https://phab.comm.dev/D6357