Gradient as colourbar

[teunbrand]

This PR aims to fix #5547.

Briefly, it adds a new argument to guide_colourbar(): display, which can be set to "gradient' to display the bar as a gradient.

In some more detail; since the raster argument was implemented as a boolean, I'd thought it best to deprecate it and use display as an enumeration instead. Possible values for display are "raster", which is the same as current raster = TRUE, "rectangles", equivalent to raster = FALSE and "gradient", the new option.

When display = "gradient", the nbins argument is used for the number of colours set in the gradient, rather than actual bins. As such, we don't need to account for the half bin-size offset for the label placement. The nbins number can be a lot less in gradient displays compared to raster/rectangle displays. As such, nbins defaults to 15 for gradients.

Prior to R 4.1.0, {grid} hadn't implemented linearGradient(), so for older R versions it throws an error saying that linearGradient is unavailabe in the R version.

No visual test has been included due to {vdiffr} not supporting gradients.

Small demo:

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

ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(colour = cty)) +
  guides(colour = guide_colourbar(display = "gradient"))

^{Created on 2023-11-29 with reprex v2.0.2}

[thomasp85]

LGTM

[teunbrand]

Thanks for the review!