Includes the inner rules when the style knows its extent. A bordered block is very often a
box with a grid inside it, and the four outer edges cannot say so — a payload that stops at
the outline reports "thin box" for a table and for a boxed blank cell alike. horizontal and
vertical are null on a cell-scoped style, which is why the region payload reads through
getRange().
Describes borders — the edges that carry a style, each as
edge style color.