feat: update layers.css to suggest correct placement for UnoCSS icons

[J-Sek]

After adding UnoCSS icon preset, the uno-icons layer it landed in the wrong place, so I was not able to apply color to the icon.

[J-Sek]

Leaving a workaround for the poor souls that find this PR looking for a hint when they cannot influence CSS layers for some reason:

initUnocssRuntime({
  defaults: {
    presets: [
      presetIcons({
        processor(props) {
          delete props.color // 👈 workaround for <icon color="..." /> not working
        },
      }),
    ],
  },
})