Interpolate gradients using OKLCH by default

[adamwathan]

This PR updates all of our gradient utilities to interpolate using OKLCH by default instead of sRGB. This results in a smoother transition between colors that preserves saturation throughout the gradient, rather than hitting the dreaded dull gray zone in between your color stops.

Here are a few examples comparing sRGB (top) to OKLCH (bottom):

We only apply a default interpolation mode when not using arbitrary values with the gradient utility.

Simplified but clear:

.bg-linear-to-r {
  background-image: linear-gradient(to right in oklch, var(--gradient-color-stops));
}

.bg-linear-[to_right] {
  background-image: linear-gradient(to right, var(--gradient-color-stops));
}

.bg-linear-[to_right_in_hsl] {
  background-image: linear-gradient(to right in hsl, var(--gradient-color-stops));
}

[trcio]

very happy I saw your tweet about this PR, would have never known about this sick color space otherwise ‼️