Parse underscore type lambdas (And +_ and -_ symbols)

[neko-kai]

Recently added to Scala 2 in: scala/scala#9605 (Under flag -Xsource:3 + kind-projector flag -P:kind-projector:underscore-placeholders typelevel/kind-projector#186)
And to Scala 3 in: scala/scala3#12378 (Under flag -Ykind-projector:underscores)

Note: Scala (with the above flags) now accepts tokens +/- followed by _ as a type lambda placeholder. Some examples w/ edge cases below:

• https://github.com/scala/scala/pull/9605/files#diff-9cc41709c754ef3f0d8f37556e954ff08ce6a8531205c710a4fa84eb59db145a
• https://github.com/lampepfl/dotty/pull/12378/files#diff-95ae2d65c0a03a3dee556e5d610eaa65bfb4d614282904302d3baed217ba659c
• https://scalacenter.github.io/scala-3-migration-guide/docs/tutorials/kind-projector.html

[dos65]

@neko-kai Will this change be included in 2.12.14 + -Xsource:3?

[neko-kai]

@dos65 Yes scala/scala#9620