…ymous type lambdas

The syntax roughly follows the [proposed new syntax for wildcards and placeholders](https://dotty.epfl.ch/docs/reference/changed-features/wildcards.html#migration-strategy) for Scala 3.2+ and is designed to allow cross-compilation of libraries between Scala 2 and Scala 3 while using the new Scala 3 syntax for both versions.

To enable this mode, add `-P:kind-projector:underscore-placeholders` to your scalac command-line. In sbt you may do this as follows:

```scala
ThisBuild / scalacOptions += "-P:kind-projector:underscore-placeholders"
```

This mode is designed to be used with scalac versions `2.12.14`+ and `2.13.6`+, these versions add an the ability to use `?` as the existential type wildcard ([scala/scala#9560](scala/scala#9560)), allowing to repurpose the underscore without losing the ability to write existential types. It is not advised that you use this mode with older versions of scalac or without `-Xsource:3` flag, since you will lose the underscore syntax entirely.

Here are a few examples:

```scala
Tuple2[_, Double]        // equivalent to: type R[A] = Tuple2[A, Double]
Either[Int, +_]          // equivalent to: type R[+A] = Either[Int, A]
Function2[-_, Long, +_]  // equivalent to: type R[-A, +B] = Function2[A, Long, B]
EitherT[_[_], Int, _]    // equivalent to: type R[F[_], B] = EitherT[F, Int, B]
```

Examples with `-Xsource:3`'s `?`-wildcard:

```scala
Tuple2[_, ?]        // equivalent to: type R[A] = Tuple2[A, x] forSome { type x }
Either[?, +_]          // equivalent to: type R[+A] = Either[x, A] forSome { type x }
Function2[-_, ?, +_]  // equivalent to: type R[-A, +B] = Function2[A, x, B] forSome { type x }
EitherT[_[_], ?, _]    // equivalent to: type R[F[_], B] = EitherT[F, x, B] forSome { type x }
```