Add support for inspecting colors to visual impairment + guide for choosing colors

[Leonardo Collado-Torres]

https://github.com/clauswilke/colorblindr has great utilities for simulating visual impairments. It is not available on CRAN or Bioconductor, which means that we cannot use it as-is in spatialLIBD. However, we want to add support to simulate the spatial plots for different visual impairments, and write a guide (vignette) detailing how to best choose colors.

We will likely add utilities for this on the interactive web applications. That way users will be able to check if the colors they selected are inclusive.