Add cross build for Scala 3

[griggt]

A first go at cross building for Scala 3.0.0-M1. Let's see what I missed or broke.

A few notes:

• The default Scala.js version in plugins.sbt was changed to 1.3.1 since this seems to be the only version that works here with all Scala versions 2.11 through 3.0.0-M1. In particular, a fix to support Scala 3 in ScalaJSJUnitPlugin was just recently merged for the 1.3.1 release (Support Scala 3 in ScalaJSJUnitPlugin. scala-js/scala-js#4285)

  I don't know what (if anything) this change will break.

• Scala 3 disallows ClassTag[Nothing] (Disallow Array[Nothing], Array[Null], ... to avoid ClassCastException in some cases scala3#1730)

  The change to ArraySeqTest.check() is the only solution I could come up with in a reasonable amount of time that worked for all supported Scala versions.

  ArraySeq.empty[T] would work for Scala 2.13 and Scala 3, but not for Scala 2.11/2.12 since the signature in this library has an AnyRef upper bound unlike the 2.13 stdlib.

  Perhaps someone has a better idea of what to do here.

• Scala.js complained about the @Test(timeout = 10000) annotation in LazyListTest when building with Scala 3:

  [error] 40 |  @Test(timeout = 10000) // scala/bug#6881
  [error]    |        ^^^^^^^^^^^^^^^
  [error]    |10000L is an unsupported argument for the JUnit @Test annotation in this position

  The error message is confusing because the L suffix was omitted but says it's unsupported. I added the L suffix, and the error went away.

Fixes #385

[SethTisue]

Your changes LGTM. Thanks for tackling this!

I had originally added a 2.13.4 upgrade here, but I'll make that a separate PR, as LazyList needs a bit of work

I'll publish a new version soon: #388