allow any method with proper signature to be used as an extractor

[erikerlandson]

This feature request stems from this discussion: #12793
At least 3 alternatives were mentioned on that discussion thread. The one I believe is most promising is to allow any function or method having an appropriate signature to be used as an extractor.

def asInt(s: String): Option[Int] = s.toIntOption

val x: Int = "4" match
    case asInt(v) => v
    case _ => 0

So any methods with valid extractor type signatures

A => Option[B]
A => Boolean
A => Option[(B, C, ...)]
// etc

can be used in an extractor matching context

[erikerlandson]

cc @armanbilge