Update the book for Scala 3 ("Dotty")

[Sergei Winitzki]

Most of the material in the book is unchanged when updating to Scala 3. However, some syntax changes need to be made, and some explanations may become simpler.

• Make sure all example code compiles and runs under Dotty.
• Perhaps make a script to extract all code into a markdown script that can be compiled with tut or equivalent, to make sure that all examples have correct syntax and are runnable.
• Revisit all examples and explanations of Scala features in light of Dotty changes. Some explanations may become simpler, others need change (implicit -> given, sealed trait -> enum, the "kind projector" plugin is unnecessary, etc.).