Summary:
`fabric/attributedstring` is a simple, cross-platfrom, react-specific implementation of attributed string (aka spanned string).
This diff is the first part of this which contains text primitives (types) and conversions.
Reviewed By: fkgozali
Differential Revision: D7748704
fbshipit-source-id: d76e31807e5ac7ab1a16fd6ee6445c59de5b89a2
Valentin Shergin