ascent property Null safety

The rise from the baseline as calculated from the font and style for this line.

This is the final computed ascent and can be impacted by the strut, height, scaling, as well as outlying runs that are very tall.

The ascent is provided as a positive value, even though it is typically defined in fonts as negative. This is to ensure the signage of operations with these metrics directly reflects the intended signage of the value. For example, the y coordinate of the top edge of the line is baseline - ascent.