Feature request: expand suite of colour manipulation functions

[teunbrand]

{scales} already has some nice colour manipulation functions like alpha() and muted() powered by {farver}. It would be convenient if this suite could be expanded to include other common tasks related to colours, so that dependencies on {prismatic} or {colorspace} could be omitted for straightforward colour manipulation. Specifically, I had the following manipulations in mind.

• col_shift() that takes a vector of colours and shifts the hue channel by some amount.
• col_lighter() that takes a vector of colours and adds to the luminance channel by some amount.
• col_darker() that takes a vector of colours and decreases the luminance channel by some amount.
• col_saturate() that takes a vector of colours and adds to the saturation channel by some amount. Alternatively, a function that does the same for the chroma channel in HCL space.
• col_mix() that interpolates between two (vectors of) colours in some proportion.

In particular, I'd have an use-case for col_mix() in tidyverse/ggplot2#5833.