Rewrite RichTextKit for iOS 18, macOS 15, and Swift 5

[danielsaidi]

RichTextKit should be rebuilt from the ground up, using the latest SwiftUI features like @Observable to improve performance, and with the various patterns that have emerged over time as part of the initial design.

This is a massive undertaking, and will most probably not be started until H2 next year. Until then, I will make very few changes to the current SDK. I will be happy to merge any PRs though.