WIP: Scala 3 Overhaul

[fabianhjr]

As an excuse to better learn cats and scala 3 I started with an overhaul.

This is still a work in progress. (Checking by building sbt pdf)

TODOS:

• Upgrade deps and address deprecations/warnings (*)
• Use given/using in lieu of implicit val
• Use given in lieu of implicit def
• Use extension in lieu of implicit class
• Use enum in lieu of sealed trait/case [class|object]
• Braceless Syntax / Significant Indent
• Implement other suggestions from:
  • https://docs.scala-lang.org/scala3/reference/contextual/type-classes.html
  • https://docs.scala-lang.org/scala3/reference/contextual/derivation.html
• More changes?
• Edit text and get a friend/colleague/someone else to proof-read

(*) Unaddressed:

warning: custom-instances.md:254:43: 
pattern's type ::[repl.MdocSession.MdocApp4.Tree[B]] is more specialized than the right hand side expression's type List[repl.MdocSession.MdocApp4.Tree[B]]

If the narrowing is intentional, this can be communicated by adding `: @unchecked` after the expression,
which may result in a MatchError at runtime.
              val left :: right :: tail = acc
                                          ^^^

[fabianhjr]

Need to revise this,

if left as val showInt then Show.apply is left as a lamdba (unevaluated) and the output of mdoc didn't match the text.

If changed to given showInt then Show.apply uses showInt as a cyclic reference and succeeds (but warns on compilation about an infinite loop)