compiler: account for Scala 2 and 3 differences in generated varargs code

[ahjohannessen]

This adds a bit of redudant overloads. The most important aspect for me now is that mima and tests pass plus a minor release can be shipped. At a later time, one can remove overloads and duplication, do a cleanup with a breaking change and bump major version or similar.

[ihostage]

@ahjohannessen @mkurz I checked this for Gradle and as I expected all is fine. The changes in twirl-compiler works fine for all supported build systems.

[ahjohannessen]

@ihostage That is great to hear :)

[mkurz]

@Mergifyio backport 1.6.x

[mergify[bot]]

backport 1.6.x

✅ Backports have been created

* [#738 [1.6.x] compiler: account for Scala 2 and 3 differences in generated varargs code (backport #737) by @ahjohannessen](https://github.com/playframework/twirl/pull/738) has been created for branch `1.6.x` but encountered conflicts

[ahjohannessen]

@mkurz Super :) Next thing could perhaps be to introduce something like:

trait TwirlCompiler {
   def compile(…)
   // perhaps not necessary
   def compileVirtual(…)
}

and do some cleanup with compile options.

[mkurz]

@ahjohannessen Pull requests always welcome :wink: