Support iPad's multitasking features (Slide Over, Split View) in Expo standalone apps.

According to the Apple documentation (https://developer.apple.com/library/content/documentation/WindowsViews/Conceptual/AdoptingMultitaskingOniPad/QuickStartForSlideOverAndSplitView.html#//apple_ref/doc/uid/TP40015145-CH13-SW1) it seems there are three criteria to have it enabled:

* Set the Base SDK to “Latest iOS,” as described in Setting the Base SDK in App Distribution Guide.

* Provide a LaunchScreen.storyboard file (instead of a .png image file as you did in iOS 7 and earlier), as described in Creating a Launch Screen File in App Distribution Guide.

* In your project’s Info.plist file, in the “Supported interface orientations (iPad)” array, declare support for all four device orientations

Request:

* Update app.json schema to allow declarative support for all four device directions

* Allow specifying a storyboard file as the splash screen or convert the existing splash png into a LaunchScreen.storyboard