[SIP 57] Add `runtimeChecked` method to replace `@unchecked` in the context that means "check at runtime"

[bishabosha]

SIP 57 is accepted for implementation: https://docs.scala-lang.org/sips/replace-nonsensical-unchecked-annotation.html

Add a new method scala.Predef.runtimeChecked that when applied to a value in match scrutinee position, turns off pattern match exhaustivity warnings, and refutable pattern binding warnings