Add dotty to cross-build

[Philippus]

I'm down to three errors on this one:

[error] -- [E007] Type Mismatch Error: /home/travis/build/scala/scala-parser-combinators/shared/src/main/scala/scala/util/parsing/combinator/PackratParsers.scala:110:39 
[error] 110 |        case in: PackratReader[_] => q(in)
[error]     |                                       ^^
[error]     |                 Found:    (in : PackratParsers.this.PackratReader[_])
[error]     |                 Required: PackratParsers.this.Input
[error] -- [E008] Member Not Found Error: /home/travis/build/scala/scala-parser-combinators/shared/src/main/scala/scala/util/parsing/combinator/Parsers.scala:667:7 
[error] 667 |    es.foldRight[Parser[List[Elem]]](success(Nil)){(x, pxs) => accept(x) ~ pxs ^^ mkList}
[error]     |    ^^^^^^^^^^^^
[error]     |    value foldRight is not a member of ES
[error] -- [E008] Member Not Found Error: /home/travis/build/scala/scala-parser-combinators/shared/src/main/scala/scala/util/parsing/combinator/syntactical/StandardTokenParsers.scala:32:15 
[error] 32 |    if(lexical.reserved.contains(chars) || lexical.delimiters.contains(chars)) super.keyword(chars)
[error]    |       ^^^^^^^^^^^^^^^^
[error]    |      value reserved is not a member of StandardTokenParsers.this.Tokens

[Philippus]

Fixed one error by adding a type annotation

[Philippus]

Managed to get compiling and testing to work, not sure (yet) what the next failure is about.

[Philippus]

Got a green build.

Closes #230 by the way.

[Philippus]

@SethTisue Do you have time to take another look?

[SethTisue]

LGTM. thanks for tackling this! pretty exciting to see the upgrade starting to work its way through the ecosystem 💪

[Philippus]

Also added to the dotty community build in scala/scala3#8604.