Set environment variable for number of ANSI colours

[mccarthy-m-g]

https://cli.r-lib.org/reference/num_ansi_colors.html

In RStudio and pkgdown colours just work, but for other formats (e.g., Quarto) the number of colours needs to be increased from the default of 8. It’s easy for a user to do this, but it might be nice to set these options/environment variables up internally.

The documentation says it can be error prone, so will need some testing to avoid problems.