Add desktop platform-related stylesheet props

[empyrical]

This PR adds the ReactPropTypes and Flow definitions for two style properties that are not useful for mobile platforms, but will be useful for desktop implementations:

• cursor - for setting what the cursor's style will be when hovering over a <View />
• appRegion - to make Views into draggable areas of a window, and also for marking children of said views non-draggable (like buttons). This is named after the --webkit-app-region property seen in Chrome apps and Electron apps

These could also be utilized by React Native Dom and React Native Web. (mapping appRegion back to --webkit-app-region)

Test Plan:

In a test project I was able to use appRegion and cursor in a StyleSheet and have flow check pass.

Release Notes:

[GENERAL] [ENHANCEMENT] [Libraries/StyleSheet/DesktopPropTypes.js] - Added Desktop Prop Types
[GENERAL] [ENHANCEMENT] [Libraries/Components/View/ViewStylePropTypes.js] - Added Desktop Prop Types
[GENERAL] [ENHANCEMENT] [Libraries/StyleSheet/StyleSheetTypes.js] - Added Desktop style flow types
[GENERAL] [ENHANCEMENT] [Libraries/StyleSheet/StyleSheet.js] - Added Desktop style flow types

[empyrical]

Looks like test_node10 failed due to a network hiccup:

yarn install v1.9.4
[1/5] Validating package.json...
[2/5] Resolving packages...
error Received malformed response from registry for undefined. The registry may be down.
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
Exited with code 1

But other than that, seemingly no regressions.

[empyrical]

Going to close this for now in favor of writing a proposal for allowing out-of-tree platforms to define style, view, etc props