Manual and identity scales for linewidth

[teunbrand]

This PR aims to fix #5050.

It does so by adding identity and manual scales, according to the patterns that were used for other aesthetics.
I could add some very basic tests, but I was unsure whether these would be necessary, as most other aesthetics also don't have such tests.

Some visual examples:

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

p <- ggplot(economics_long, aes(date, value01, group = variable))

p + geom_line(aes(linewidth = variable)) +
  scale_linewidth_manual(values = c(3,2,1,2,3))

p + geom_line(aes(linewidth = rescale(date))) +
  scale_linewidth_identity(guide = 'legend')

^{Created on 2023-01-06 by the reprex package (v2.0.1)}

[thomasp85]

LGTM