Ingest plain patterns as lists in layer()

[teunbrand]

This PR to the RC branch aims to fix #5593.

Briefly, instead of coercing a plain pattern to a list in Geom$use_defaults(), it does so in the layer() function.
This has two consequences:

1. Other internals, like GuideLegend$get_layer_key() or Geom$use_defaults() don't have to deal with plain patterns.
2. If a pattern is provided to another aesthetic than 'fill', it is also coerced to a list, opening up 'fill'-variant aesthetics for patterns. For example, a package might use a 'fill_box' aesthetic for a geom.

Reprex from #5593:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

grad <- grid::linearGradient(pal_viridis()(10))

ggplot(mpg, aes(factor(cyl), colour = "fixed")) +
  geom_bar(fill = grad)

^{Created on 2023-12-18 with reprex v2.0.2}

[thomasp85]

LGTM

[teunbrand]

Thanks!