Solve some oklaba inconsistencies

[pablo-lua]

Objective

• Even if we have Laba and Oklcha colorspaces using lightness as the L field name, Oklaba doesn't do the same
• The shorthand function for creating a new color should be named Oklaba::lab, but is named lch

Solution

• Rename field l in Oklaba to lightness
• Rename Oklaba::lch to Oklaba::lab

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Changelog

Changed

• Changed name in l field in Oklaba to lightness
• Changed method name Oklaba::lch to Oklaba::lab

Migration Guide

If you were creating a Oklaba instance directly, instead of using L, you should use lightness

// Before
let oklaba = Oklaba { l: 1., ..Default::default() };

// Now
let oklaba = Oklaba { lightness: 1., ..Default::default() };

if you were using the function Oklaba::lch, now the method is named Oklaba::lab