Allow `fill` and `pattern_fill` to support gradient/pattern fills (in addition to color fills)

[trevorld]

• Development version of {gridpattern} now allows gradient/pattern fills in many "geometry" patterns
• {ggplot2} v3.5.0 now allows gradient/pattern fills
• Should be straightforward to allow both in {ggpattern} to support gradient/pattern fills
• Note use of gradient/pattern fills will require R v4.2+ and a graphics device which supports the gradient/pattern fill feature.
• Use of color fills will continue to work on a greater variety of R versions and graphics devices...