generated domain classes: add unapply to named tuples for easy pattern matching

[maltek]

the new test file demonstrates how this unapply method returning a named tuple enables pattern matching on the domain classes with the familiar syntax from matching on case classes:

  "named tuple unapply" should {
    "destructure a Song node" in {
      val heyBoDiddley = NewSong().name("HEY BO DIDDLEY").songtype("cover").performances(5)
      val boDiddley    = NewArtist().name("Bo_Diddley")
      val gratefulDead = GratefulDead.from(_.addEdge(src = heyBoDiddley, dst = boDiddley, Writtenby.Label))

      inside(gratefulDead.song.toBuffer) { case mutable.Buffer(Song(name = name, songtype = Some("cover"), performances = Some(5))) =>
        name shouldBe "HEY BO DIDDLEY"
      }
      inside(gratefulDead.song.head) { case Song(name = name) =>
        name shouldBe "HEY BO DIDDLEY"
      }
    }

    "destructure an Artist node" in {
      val garcia                  = NewArtist().name("Garcia")
      val Artist(name = "Garcia") = garcia
    }
  }

Now, we have a DSL that for filtering and extracting values from iterators. I don't aim to replace it here, but to enhance it. I think this feature here will shine in e.g. places (e.g. lowering tests) where you want to check a whole lot of values of the same node at once, and when you're using pattern matching already to replace an if on some match arms with a pattern (i.e. turn case x: X if x.foo == ... into case X(foo = ...).

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This PR is made a bit awkward because named tuples are a Scala 3 feature and flatgraph still supports Scala 2. My strategy here was to make this feature optional: by default the generated code works with both Scala 2 and 3. But the code generator can be told to to use Scala 3 features, and then you get these named tuples. (You could even run the generator for the same schema twice, once to put code into src/main/scala2 and once into src/main/scala3 if you wanted a project to be compatible with either version and still make use of this feature in code that's specific to Scala 3.)

The interesting diff of this PR is in domain-classes-generator/src/main/scala/flatgraph/codegen/DomainClassesGenerator.scala and tests-scala3/src/test/scala/flatgraph/traversal/NamedTupleUnapplyTests.scala

We could also completely drop Scala 2 - we don't use it anymore in joern. But I'd feel a little bad doing that just for this feature.

[ml86]

From my point of view we could drop Scala 2 support. This would also make our life easier to get a Bazel build for this repo going. Scala 3 is 5 years old by now. I dont think somebody could complain about not having enough time to switch and even if so Scala 3 code is supposedly callable from Scala 2 code...