Added NSFaceIDUsageDescription Key to InfoPlist

[fhasse95]

This PR adds the required NSFaceIDUsageDescription key to the Info.plist.

When I added the BiometricAuthenticationPlayground in #116, I tested it on the iOS Simulator and on an Android device, where everything worked as expected. However, after installing the latest version of Skip Showcase from the App Store, I discovered that the app crashes on physical iOS/iPadOS devices when attempting Face ID authentication. This is because accessing Face ID on physical iOS/iPadOS devices requires the NSFaceIDUsageDescription key in the app's Info.plist, which I unfortunately hadn't realized earlier.

To help prevent others from running into the same issue, I also updated the README in SkipKit in skiptools/skip-kit#39 to mention that this Info.plist entry is required when using biometric authentication on iOS/iPadOS.

Sorry for the inconvenience!

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