feat(core): introduce special symbols for custom variants

[antfu]

• close Ability to wrap rules (E.g. for @supports feature query) #3862
• resolve 能实现一个规则，多个样式么？ #2175
• resolve How to write a media-query custom rule? #17
• resolve Best way to implement scrollbar utilities #295

Combining with #3884, now it's able to create complex selectors within a single rule:

For example:

rules: [
  [/^button-(.*)$/, function* ([, color], { symbols }) {
    yield {
      background: color
    }
    yield {
      [symbols.selector]: selector => `${selector}:hover`,
      background: `color-mix(in srgb, ${color} 90%, black)`
    }
  }],
]

Will generate multiple CSS rules:

.button-red {
  background: red;
}
.button-red:hover {
  background: color-mix(in srgb, red 90%, black);
}

Learn more here: https://unocss.dev/config/rules#special-symbols

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