Support layer attribute on <style>

[KazariEX]

convert

<style layer="name">
  .foo {
    color: aqua;
  }
</style>

to

@layer name {
  .foo {
    color: aqua;
  }
}

It's helpful for writing general components. When it is necessary to apply all styles within the component to the @layer block, it can reduce the indentation by one level.