Bivariate colour interpolation (`pal_ramp_bibariate()`)

[mccarthy-m-g]

New function for creating bivariate colour palettes that can be used with the biscale package. Main points of consideration are:

• Should the function take 4 colours as arguments (one for each corner of the palette), or 3 colours as arguments (with the 4th being interpolated based on the x and y colours)?
• Should the number of dimensions be infinite, or should they be restricted? (this is more a practical question based on how well I can program something that generalizes well to higher dimensions, or if I can only create something nice for smaller numbers of dimensions)