Introduce UITextView support

[DivineDominion]

Imported from CleanCocoa/DeclarativeTextKit#2

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Since UITextView and NSTextView share most of the underpinnings, including being backed by UTF-16 NSStrings, adding UIKit support should consist of either

• adding an adapter like NSTextViewBuffer but for UITextView,
• use a TextViewBuffer (sans prefix) for both.

With conditional compilation, #if os(macOS) around the NSTextView property declaration and initializer should do the trick iff the API really is exactly the same.

• Add iOS, iPad, ... as deployment targets
• Re-use or add tests for UITextView