New error handling

[odersky]

New language feature: experimental.errorHandling. It needs -Yexplicit-nulls to be also set

This enables the following:

• Add maybe types T? and T ? E.
• Add maybe prompt and postfix e? operator
• Implement subtype rules
• Implement erasure rules
• Special pattern-matching rules for Ok and Err extractors

Have you relied on LLM-based tools in this contribution?

I have relied on LLMs to

• make proposals how to adapt the logic in Space.scala to the new types
• generate some of the tests from the tests we had for Option
• give some hints for debugging.

[noti0na1]

While doing a quick test with this PR locally, I find some issues with type erasure and type bounds, worth adding some tests:

// type erasure and generic signature would give different results, so calling f in Java may have issue
def f(x: String?): String? = x

def f1(x: String?): String = x match
  case Ok(y) => y
  case null => "none"

def f2[T <: String](x: T?): String = x match
  case Ok(y) => y
  case null => "none"

def f3[T <: String?](x: T): String = x match
  case Ok(y) => y
  case null => "none"

val s: String? = "a"
val b = s.isEmpty