Scala 3

[keynmol]

There are some non-trivial changes that I had to make, so let's have a discussion here.

1. Add Scala 3 to cross build
2. Exclude Scala 3 Native combination from tests (see scala-native Scala 3 support scalatest/scalatest#2097)
3. Adjust flags for the Scala.js on Scala 3 combination
4. Update SBT to 1.6.x
5. Update Scala.js to 1.9.0 (can be dropped down to 1.7.0, but not lower as that's the IR version that Dotty starts out with)
6. Change do...while to recommended construct in https://docs.scala-lang.org/scala3/reference/dropped-features/do-while.html
7. Remove auto-application issue on Scala 3 (see https://docs.scala-lang.org/scala3/reference/dropped-features/auto-apply.html#inner-main) - this is not binary compatible, as I removed case from Arg and Fragment classes

[keynmol]

Issues like

[error] 134 |        val bar = !# / "asdf" / Arg[Int] / Arg[Boolean]
[error]     |                                           ^^^^^^^^^^^^
[error]     |            method apply in object Arg must be called with () argument

[lolgab]

@keynmol scalatest 3.2.12 was released with native0.4_3 artifacts :-)

[keynmol]

@tindzk seems like Travis pipelines are not running - if you want I can add a GH workflow for tests, you'll just need to enable GH actions for this repository.

[DeamonDev]

So what about that PR? I would like to use this library together with Tyrian and for that case I definitely need Scala 3 version of trail.