Support `@media` nesting

Spec: https://drafts.csswg.org/css-nesting-1/#conditionals

.foo {
  display: grid;

  @media (orientation: landscape) {
    grid-auto-flow: column;

    @media (min-width > 1024px) {
      max-inline-size: 1024px;
    }
  }
}

/* equivalent to */
  .foo { display: grid; }

  @media (orientation: landscape) {
    .foo {
      grid-auto-flow: column;
    }
  }

  @media (orientation: landscape) and (min-width > 1024px) {
    .foo {
      max-inline-size: 1024px;
    }
  }

Read the linked CSS Nesting Level 1 specification and compare its conditional examples with the requested equivalence. Then inspect lightningcss's parser and compiler entry points to locate existing nesting and @media handling. Done means nested @media rules are transformed into the equivalent conditional rules shown in the issue, with coverage for nested conditions.