add layer example

[jantimon]

This PR adds examples demonstrating CSS @layer support in next-yak, showcasing how layers can be used to manage style precedence and cascade in a more structured way

• Example: Added @layer declarations in global CSS
• Example: Implemented layered styles in a styled component

CSS @layer is a powerful addition to CSS that allows developers to explicitly manage the cascade and control specificity. When combined with next-yak, it offers several key benefits:

• Layers provide a way to control specificity without relying on selector specificity or !important
• The order of layer declarations determines their precedence, making style overrides more predictable

Example:

@layer global, components, theme;

const Button = styled.button`
  @layer components {
    /* Base styles */
    background: white;
  }
  @layer theme {
    /* Theme-specific overrides */
    ${({ theme }) => theme.dark && css`
      background: black;
    `}
  }
`;