…eholder syntax

This change allows `kind-projector` plugin to rewrite `+_` and `-_` tokens to type lambdas,
in line with proposed syntax for Scala 3.2 in http://dotty.epfl.ch/docs/reference/changed-features/wildcards.html

When used in conjunction with `-Xsource:3` this will let the user use `?` for wildcards and `_` for placeholders, letting the user cross-compile the same sources with Scala 3 with `-source:3.2` flag.

This change is not source breaking since currently `+_` and `-_` fail to parse entirely,
this change also does not allow the user to declare types with these names without backticks,
they can only be used as part of a type tree.

…ption to existing code