Add pattern completion for unapply contexts

[natsukagami]

Fixes #19972.

Add pattern completion for Unapply tree contexts.

A typical example would be

optionList match
    case List(S@@)

which should be prompted Some(value), due to List.unapplySeq expecting Option[T] patterns as arguments.

• Add unapply pattern completion test
• Add pattern completion based on the Unapply argument type

[rochala]

I left 2 comments, and if you could also add test which checks deeper nesting levels. Other than that it looks fine to me

Will this also work for multiple nested levels ?

null match
  case Some(Some(Some(Som@@))))

[rochala]

I'm not sure whether we should still show synthetic constructor / apply completions in this case. They are invalid here.

Completions for completions is a different story though, as we have no logic to detect whether it has nested members yet, so this is another improvement that could be made in the future.

If you want to remove Some[A](value: A): Some[A] I suppose it can be achieved by adding unapply cases here:

//file: Completions.scala
  private lazy val shouldAddSnippet =
    path match
      /* In case of `method@@()` we should not add snippets and the path
       * will contain apply as the parent of the current tree.
       */
      case (fun) :: (appl: GenericApply) :: _ if appl.fun == fun =>
        false
      case _ :: (withcursor @ Select(fun, name)) :: (appl: GenericApply) :: _
          if appl.fun == withcursor && name.decoded == Cursor.value =>
        false
      case (_: (Import | Export)) :: _ => false
      case _ :: (_: (Import | Export)) :: _ => false
      case _ => true