Add tvOS support

[mpospese]

Intro

tvOS uses many of the same UIKit components as iOS, so we should expand Y—CoreUI to support tvOS.

Task

• add .tvOS(.v14) platform to Swift package
• Port YCoreUI to compile on tvOS
• Port YCoreUI to pass unit tests on tvOS
• Unsupported functionality (if any) should be excluded from tvOS using #if !os(tvOS) or if os(iOS) (keep future watchOS support in mind)
• Functionality that is supported but does not work (if any) should be excluded from tvOS

AC

• Project compiles for iOS and tvOS without warnings
• SwiftLint returns 0 violations
• All unit tests pass on both platforms
• 100% documentation coverage of all public interfaces as determined by Jazzy