[HSLuv][1] is a color space that is like HSL, but adjusts the lightness
component so that each color within the H-S plane is perceptually
uniform for each value of L.

`hsluv()` is not a valid function in CSS, but there is a new spec ([CSS
Color 4][2]) that introduces `lab()` and `lch()`, so `hsluv()` is an
extension to that idea.

In order to highlight `hsluv()` functions as colors in Vim:

* You will need to configure Sass to recognize `hsluv()` as a function.
  You can make use of my [`sass-hsluv`][3] library to do this. (There
  are instructions in the README for how to configure Webpack and other
  bundlers.)
* You will need to install two Vim plugins of mine: [`vim-hsluv`][4] and
  [`vim-css-hsluv`][5] (the latter adds `hsluv()` to a syntax
  highlighting group; the former has HSLuv color conversion functions).

[1]: https://www.hsluv.org/
[2]: https://www.w3.org/TR/css-color-4/#specifying-lab-lch
[3]: https://git.sr.ht/~mcmire/sass-hsluv
[4]: https://git.sr.ht/~mcmire/vim-hsluv
[5]: https://git.sr.ht/~mcmire/vim-css-hsluv

Originally I was going to introduce this as a separate plugin, but it
belongs here.