Add per-range support for line height

[gskinner]

Line height is a multiplier in Flutter, and is calculated per line based on the largest text in the line. In XD it is a fixed value for all lines in a field. This may result in significant differences when displaying multiline text having multiple text sizes.

It may be possible to improve on this by calculating a line height for each TextSpan individually, though it would require knowing what the "default" line height is.

Ex.
spanHeight = (xdNode.lineSpacing ?? _calculateDefaultHeight()) / (span.fontSize)

The unknown is calculating the default height if linespacing is 0. It would also be nice to use that to omit the line spacing code if a range is using the default.