One blank line between declarations

[ValentinWalter]

This adds a new configuration option (oneBlankLineBetweenDeclarations) to make the pretty printer add a blank line between declarations that are either multiline, or have differing declaration modifiers. This works at the file scope as well. Here's an example:

typealias A = Int
typealias B = String
class A {
  private static var x1: Int
  private static var x2: Int
  var y1: Int
  var y2: Int
  private var z1: Int
  private var z2: Int
  static var w1: Int
  static var w2: Int
}

Becomes this:

typealias A = Int
typealias B = String
+
class A {
  private static var x1: Int
  private static var x2: Int
+
  var y1: Int
  var y2: Int
+
  private var z1: Int
  private var z2: Int
+
  static var w1: Int
  static var w2: Int
}