Allow _ as a type lambda placeholder in -Ykind-projector:underscores compatiblity mode

[neko-kai]

Also allow through +_ and -_ only in that mode, these variance markers are ignored since we infer variance.

Fix partially #5379

Related issues and PRs: typelevel/kind-projector#120 typelevel/kind-projector#188 scala/scala#9605

note

What I did in Settings.scala to allow -Ykind-projector to parse without an argument is likely suboptimal, but at least it's a very small diff...

[neko-kai]

Note: #5379 is not fully addresssed because there is an unresolved question on what should F[_[_]] translate to.

SIP mentions that analogy with value-level placeholders is a reason to use _ for type lambdas as well.

However, in Scala 2 kind-projector will translate the above to [G[_]] =>> F[G], but a similar value-level lambda f(_(_)) will translate to f(g, a) => g(a)).

Therefore this change repurposes Ykind-projector machinery to allow cross-compilation with a subset of new syntax and leaves the above question to a future 'proper' implementation of the SIP.