`oklch` in CSS custom properties is not transformed

When using `esbuild` with `target` set to older browsers, `oklch` functions used in custom property values are not being transformed.

```css
--color: oklch(69% 0.1 50); /* should become #cd8962 */
```

See [esbuild playground](https://esbuild.github.io/try/#YgAwLjIzLjEAewogIHRhcmdldDogWwogICAgImNocm9tZTExMCIsCiAgICAiZmlyZWZveDExMCIKICBdCn0KAGUAZW50cnkuY3NzAGJvZHkgewogIC0tY29sb3I6IG9rbGNoKDY5JSAwLjEgNTApOwogIGNvbG9yOiBva2xjaCg2OSUgMC4xIDUwKTsKfQ). Compare this with [lightningcss playground](https://lightningcss.dev/playground/index.html#%7B%22minify%22%3Afalse%2C%22customMedia%22%3Afalse%2C%22cssModules%22%3Afalse%2C%22analyzeDependencies%22%3Afalse%2C%22targets%22%3A%7B%22chrome%22%3A7208960%2C%22firefox%22%3A7208960%7D%2C%22include%22%3A0%2C%22exclude%22%3A0%2C%22source%22%3A%22body%20%7B%5Cn%20%20--color%3A%20oklch(69%25%200.1%2050)%3B%5Cn%20%20color%3A%20oklch(69%25%200.1%2050)%3B%5Cn%7D%5Cn%22%2C%22visitorEnabled%22%3Afalse%2C%22visitor%22%3A%22%7B%5Cn%20%20Color(color)%20%7B%5Cn%20%20%20%20if%20(color.type%20%3D%3D%3D%20'rgb')%20%7B%5Cn%20%20%20%20%20%20color.g%20%3D%200%3B%5Cn%20%20%20%20%20%20return%20color%3B%5Cn%20%20%20%20%7D%5Cn%20%20%7D%5Cn%7D%22%2C%22unusedSymbols%22%3A%5B%5D%2C%22version%22%3A%22local%22%7D).

---

I came across a similar old issue (https://github.com/evanw/esbuild/issues/2566) which was closed as "won't implement" with the following rationale:

> CSS variables are arbitrary token sequences and don't have specific semantics. You can also read the value of a CSS variable from JavaScript so doing what you suggested could introduce bugs.

Technically this may be correct, but I wonder how useful it is in practice. `oklch` is a built-in CSS function which will always compute to a [``](https://drafts.csswg.org/css-color-5/#typedef-color) value, so I would expect it to be transformed no matter where it used.

It is extremely common to assign color values to CSS variables to make them easier to reference later, whereas I have never once seen a color variable being read from JS in a way that relies on its exact verbatim value. The current behavior makes esbuild unreliable for CSS transformations.