Add GenLens for Scala 3, redirect to Focus

[julien-truffaut]

/cc @asjad02

Macros

• split macros module into Scala 2 et Scala 3 files
• Implement GenLens for Scala 3 using Focus

  Monocle/macro/src/main/scala-3.x/monocle.macros/GenLens.scala

  Lines 6 to 8 in 61ceedb

  	object GenLens {
  	def apply[A] = new MkFocus[A]
  	}

• Move all macro tests to macros modules

Generic

• move generic tests to generic module

Tests

• enable Scala 3
• remove all usage of GenX in test module
• remove shapeless
• disable StringsSpec, there was an issue with ScalaCheck and refined (I will investigate in a separate ticket)

Todo later

• re-enable StringsSpec
• Implement GenPrism using as from Focus [Focus] add as for enumerations #1028
• Implement GenIso problably tricky

The alternative for macros is to wait for Scala 2.13.5 which should allow to cross compile macro to Dotty. However, I am bit worried how our build will handle it (2.13, 3.0 and 2.13 compiling to 3.0).

[kenbot]

Lgtm.

I think at some point we will have to revisit the GenXXX and Focus tests and unify them into something that makes sense. But it doesn't have to happen in this PR, I think, it can come later.