Instead of:

    val x: A with B = new A with B {}

One can now write:

    val x: A & B = new A with B {}

However mixing `&` with other infix operators is not allowed, because
unlike Scala 3, we do not take operator precedence into account, cf scala#6147.

This implementation is a bit more restrictive than the Scala 3 one which
allows shadowing the built-in `&` with your own `&` type operator, but
this cannot be done with the simple parser-based approach of this PR.