Add support for Swift 5.3 and older platforms

• make TreeBuilder compile conditionally to support Swift 5.3
• drop platforms in Package.swift to support older platforms
• add @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *) to TreeDifference and related types
• move OutlineViewTreeDataSource into separate module called TreeUI

First Release of Tree

v0.1

create readme