Support gradients in color sliders for RGB/HSL

[Kyren223]

In the RGB/HSL it's now just a weird white color, instead, have a gradient color like the one the for interpolating between 2 colors has.

For reference, see ccc.nvim, the idea is for each value (R, G, B, H, S, L, etc) it'll show the color range.
For example if we have RGB with R 60, G 200, B 30, the R values will interpolate from (0, 200, 30) to (255, 200, 30), while G is from (60, 0, 30) to (60, 255, 30) etc (same for HSL and any other color spectrum).

[max397574]

definitely think this would be something useful
would be happy to review a pull request for this

I myself won't have the time to work on this in the near future

[Kyren223]

Unfortunately I am also busy (and never really worked on a nvim plugin before), I am ok with waiting for you or someone else to do it.

Also another related idea, being able to control how wide (in chars) the sliders are, so the color transition would look smoother, I find the current version a little bit too short (I think ccc.nvim is around 30% longer) and I find the gradient to maybe be a little too wide, so it might be nice to have a configurable length.