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:
nix-community/stylix#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.

[make-42]

Since people might want to adjust lightness for the dark and light schemes separately it might be worth it to have two separate arguments for each (especially since increasing the lightness past 1 for the dark scheme does nothing and decreasing the lightness past -1 for the light scheme does nothing as well)

[make-42]

Here are some examples

Lightness scale dark: 0 (standard)

Lightness scale dark: -0.05

Lightness scale dark: -0.2

Lightness scale dark: 0.2