Use visibleRect to compute visible character ranges; add textRange(in:) helper

- Derive visibleCharacterRanges and accessibilityVisibleCharacterRange from
  contentView.visibleRect via NSTextLayoutManager.textRange(in:) instead of
  viewportRange for more accurate results.
- Add NSTextLayoutManager.textRange(in:) to map a rect to a text range.
- Minor cleanup: rename local var to viewportLayoutController and make
  liveResizeLayoutSuppression private.