[css-fonts-4] Make color interpolation method optional in palette-mix()

[cdoublev]

<color-interpolation-method> is defined as optional in prose for <palette-mix()>:

When not specified otherwise the is in oklab as recommended by CSS Color 5 § 7.1 Color Space for Interpolation.

But not in its value definition:

palette-mix(<color-interpolation-method> , [ [normal | light | dark | <palette-identifier> | <palette-mix()> ] && <percentage [0,100]>? ]#{2})

I suppose this is an oversight.

[svgeesus]

I think the prose is incorrect, because in discussions it was clear that palette-mix() was closely based on color-mix() where the color-interpolation-method is not optional.

But I think there will be ongoing discussion on this, so let's leave this PR open while that happens.

[svgeesus]

Further discussion aligned palette-mix() with color-mix(), so the color interpolation method is (per grammar) not optional.

Closing this PR therefore and I will remove the prose mention of it being optional.

[svgeesus]

Fixed by 1a415a2