Add Swift function builder support like SwiftUI uses

[Zev Eisenberg]

Tired:

stack.stackable.add([
      logo,
      30,
      "Example Views",
      10,
      cells
          .outset(to: view)
          .margins(alignedWith: stack),
      UIStackView.stackable.hairlines(around: cells)
          .outset(to: view),
      20...,
      "Copyright Rightpoint",
  ])

Wired:

stack.stackable.add {
      logo
      30
      "Example Views"
      10
      cells
          .outset(to: view)
          .margins(alignedWith: stack)
      UIStackView.stackable.hairlines(around: cells)
          .outset(to: view)
      20...
      "Copyright Rightpoint"
  }