Make SIP-62 - betterFors a standard feature

[KacperFKorban]

• Deprecate experimental language import
• Make betterFors conditional on -source >= 3.7 instead
• Drop the experimental.betterFors import from tests

[KacperFKorban]

While working on this, I discovered an unhandled/bug case that apparently isn't used anywhere in the open community build – nested given bindings inside of a pattern of an alias. I added a quickfix for it.
e.g.

for
    x <- Option(23 -> "abc")
    (a @ given Int, b @ given String) = x
    _ <- Option(1)
  yield
    assert(summon[Int] == 23)

betterFors generated:

Option.apply[(Int, String)](ArrowAssoc[Int](23).->[String]("abc")).flatMap
  [Nothing]((x: (Int, String)) =>
  {
    val $2$: ((Int, String), Int, String) =
      x:(Int, String) @unchecked match 
        {
          case 
            $1$ @
              Tuple2.unapply[Int, String](given a @ _:Int,
                given b @ _:String)
            => Tuple3.apply[(Int, String), Int, String]($1$, a, b)
        }
    val $1$: (Int, String) = $2$._1
    val a: Int = $2$._2
    val b: String = $2$._3
    Option.apply[Int](1).map[Nothing]((x$1: Int) =>
      x$1:Int @unchecked match 
        {
          case _ => assert(summon[Int](/* missing */summon[Int]).==())
        }
    )
  }
)

After the fix, it generates:

Option.apply[(Int, String)](ArrowAssoc[Int](23).->[String]("abc")).map[
  ((Int, String), (Int, String))]((x: (Int, String)) =>
  {
    val $2$: ((Int, String), Int, String) =
      x:(Int, String) @unchecked match 
        {
          case 
            $1$ @
              Tuple2.unapply[Int, String](given a @ _:Int,
                given b @ _:String)
            => Tuple3.apply[(Int, String), Int, String]($1$, a, b)
        }
    val $1$: (Int, String) = $2$._1
    val a: Int = $2$._2
    val b: String = $2$._3
    Tuple2.apply[(Int, String), (Int, String)](x, $1$)
  }
).flatMap[Unit]((x$1: ((Int, String), (Int, String))) =>
  x$1:(x$1 : ((Int, String), (Int, String))) @unchecked match 
    {
      case 
        Tuple2.unapply[(Int, String), (Int, String)](x @ _,
          Tuple2.unapply[Int, String](given a @ _:Int, given b @ _:String)
          )
        =>
        Option.apply[Int](1).map[Unit]((x$1: Int) =>
          x$1:Int @unchecked match 
            {
              case _ =>
                if a.==(23).unary_! then
                  scala.runtime.Scala3RunTime.assertFailed() else ()
            }
        )
    }
)

[odersky]

Great to have this for 3.7

[odersky]

Needs to resolve a conflict., otherwise ready to go in.

[tgodzik]

@KacperFKorban The oficial decision was to have it in preview mode for 3.7 and only stabilize in 3.8 to be on the safe side. Could add it to preview in a follow up? Sorry for raising it just now, I think this wasn't communicated clearly.