Summary:
Adds a sparse `CSSDeclaredStyle` structure to represent the collection of declarations from the user, before being further processed/computed. This will later be procssed into computed style.
Also fixes up some bad naming wrt specified value and declared value.
Changelog: [Internal]
Reviewed By: joevilches
Differential Revision: D53696422
fbshipit-source-id: 4258c86a29a0b2251c969b299b5b4703c06696db