Fix extra-line metrics via extraLineFragmentAttributes (Simplify the FB15131180 workaround)

- STTextLayoutFragment now accepts defaultTypingAttributes and exposes
  @objc(extraLineFragmentAttributes) to feed correct font metrics to the
  typesetter.
- Determine paragraph style using characterRange.isEmpty fallback.
- Remove NSTextLayoutFragment+Helpers.stTypographicBounds and all special-casing
  for extra line fragments in selection/caret drawing.
- Compute content height by enumerating text segments at the document end;
  width from last layout fragment; applied to sizing and viewport relocation
  on both AppKit and UIKit.