Add support for `Arrows::width()`

[nstoddard]

Is your feature request related to a problem? Please describe.
Sometimes arrows can be hard to see (e.g. when they're along the X or Y axis) so it would be nice to be able to increase their width so they're more visible.

Describe the solution you'd like
Add Arrows::width(), which would work like Line::width() and similar.

Describe alternatives you've considered
Arrows::highlight(true) helps, but it would be nice to have more control than this offers.

Additional context