feat: add lightness option

[make-42]

Here is a proposed feature,

This PR would add a lightness option to adjust lightness of the outputted color scheme.

Here's a graph of how the lightness option affects the outputted lightnesses: https://www.desmos.com/calculator/uagjexbi1x

This feature would allow stylix to have a lightness option if it switches to matugen as its color scheme generation engine: https://github.com/danth/stylix/pull/892

(in the original PR the lightnesses were adjusted post matugen but it's a bit hacky so it would be better to implement it upstream)

Thank you.