[Scala 3] Make instances Serializable

[TimWSpence]

Currently failing in CI

Further investigation has revealed that it's actually a problem for all the derived instances. Example failure:

> Exception: java.io.NotSerializableException: cats.derived.ShowPrettySuite$$anon$3
    at munit.ScalaCheckSuite.propToTry(ScalaCheckSuite.scala:98)
Caused by: java.io.NotSerializableException: cats.derived.ShowPrettySuite$$anon$3

Decompiling that class:

javap core/.jvm/target/scala-3.1.2/test-classes/cats/derived/ShowPrettySuite\$\$anon\$3.class
Compiled from "ShowPrettySuite.scala"
final class cats.derived.ShowPrettySuite$$anon$3 implements scala.deriving.Mirror$Sum {
  public cats.derived.ShowPrettySuite$$anon$3();
  public int ordinal(cats.derived.TestDefns$IntTree);
  public int ordinal(java.lang.Object);
}

So the issue is that if a type doesn't have a companion object then the Mirror is derived as an anonymous class that is not Serializable.

The most obvious fix is to get Dotty to make these anonymous instances Serializable

[TimWSpence]

Tested locally against dotty 438dfb024faa91f4239f346fa4f496a20c5e3d15 (with the fix from scala/scala3#15391) and it resolves the issue 🎉