Cleanup CSSKeyword (#49155)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49155

This does some code cleanup for CSS keywords to reduce boilerplate, duplication, better isolate namespace, fix a typo, and ensure we get a warning (unused variable) if we miss handling a defined keyword.

We technically don't need `CSSKeyword` at all anymore, and don't need to overlay each keywords values to be the same, though having a pattern where each keyword set uses ordinal values from CSSKeyword forces folks to look and add the enum to the list, and include the header defining the data types for keyword sets, instead of each set looking a little magic.

Changelog: [Internal]

Reviewed By: lenaic

Differential Revision: D69083181

fbshipit-source-id: b2764e87c2a127d73f816327c4edd45151ea8d82