[v4] Support named alpha values in theme as color modifiers

[emmbm]

Named opacity values were previously supported as color alpha modifiers. They have been removed by #14278, but the rationale is - i humbly think - questionable and there remains a case for the relevance of reusable named alpha values.

Supporting some form of theme values for alpha modifiers through a set of variables other than opacity could prevent the confusion risks mentioned in the linked PR all while enabling a better DX with named vars that can be customized independently of their underlying value and prove relevant when designing systems with that expect standardized background or text fading independent of colors.

For example:

@theme {
  --color-primary: oklch(0.58 0.2 275);
  --alpha-soft: 0.15;
}

<button class="bg-primary/soft">Some call-to-action</button>

Sure, one currently could resort to passing the full var function as an arbitrary value (...-my-color/[var(--opacity-foo)]), but this is somewhat cumbersome compared to the syntax that comes with supported theme values (...-my-color/foo).