Currently the postprocess hook only allow to modify the existing rule object, not append more rules or remove the existing rule. However, sometimes we need to transform a rule to multiple rules. For example, color fallback:

/* Before post-processing: an OKLCH color only */
.text-shadow-color-blue-300 { --un-text-shadow-color: oklch(0.8 0.11 256); }

/* After post-processing: add RGB fallback */
.text-shadow-color-blue-300 { --un-text-shadow-color: oklch(0.8 0.11 256); }
@supports not (color: oklch(0 0 0)) { .text-shadow-color-blue-300 { --un-text-shadow-color: #8ec1ff; } }

Since PostCSS doesn't work well with UnoCSS (--at-apply is handled AFTER PostCSS!), built-in post-process hook is important for such transformations.

This PR allows the config.postprocess hook functions to return:

• false: to remove the rule
• Other falsy values: to keep the rule object (maybe mutated by the function, the original behavior)
• An object: to replace the rule with a new rule object
• An array: to replace the rule with an array of rules