Forward-port the content of the Scala 2 book into the Scala 3 book

[julienrf]

In an effort to unify the Scala 2 and Scala 3 documentation, we would like the current Scala 3 Book to become a complete documentation of Scala, describing the whole language and detailing the specificities of Scala 2 and Scala 3 when needed. Ultimately, it will be renamed to just “Scala Book”.

To get there, we must carefully read every page of the Scala 2 book and check that its content is fully covered by its corresponding page in the Scala 3 book. If something is missing, that should be added to the Scala 3 book. And if there is no corresponding page in the Scala 3 book, it should be added to it (possibly marked as Scala 2 specific if needed).

See also #2481 for a similar issue on the unification of the Scala 2 and Scala 3 documentation.

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Here is a checklist that tracks our progress.

• Introduction
• Prelude꞉ A Taste of Scala
• Preliminaries
• Scala Features
• Hello, World
• Hello, World - Version 2
• The Scala REPL
• Two Types of Variables
• The Type is Optional
• A Few Built-In Types
• Two Notes About Strings
• Command-Line I/O
• Control Structures
• The if/then/else Construct
• for Loops
• for Expressions
• match Expressions
• try/catch/finally Expressions
• Scala Classes
• Auxiliary Class Constructors
• Supplying Default Values for Constructor Parameters
• A First Look at Scala Methods
• Enumerations (and a Complete Pizza Class)
• Scala Traits and Abstract Classes
• Using Scala Traits as Interfaces
• Using Scala Traits Like Abstract Classes
• Abstract Classes
• Scala Collections
• The ArrayBuffer Class
• The List Class
• The Vector Class
• The Map Class
• The Set Class
• Anonymous Functions
• Common Sequence Methods
• Common Map Methods
• A Few Miscellaneous Items
• Tuples
• An OOP Example
• sbt and ScalaTest
• The most used scala build tool (sbt)
• Using ScalaTest with sbt
• Writing BDD Style Tests with ScalaTest and sbt
• Functional Programming
• Pure Functions
• Passing Functions Around
• No Null Values
• Companion Objects
• Case Classes
• Case Objects
• Functional Error Handling in Scala
• Concurrency
• Scala Futures
• Where To Go Next