Support @layer keyword

[earlAchromatic]

Environment

• clean-css version: 5.3.1
• node.js version: 16
• operating system: MacOS

Configuration options

using grunt-contrib-cssmin (no config)

Input CSS

@layer customlayername {
    .custom-rule {
        background: red;
    }
}

Actual output CSS

@layer customlayername {
        background: red;
}

And error:

Unexpected '}' at .tmp/css/app.css.,Unexpected '}'

[jakubpawlowicz]

The fix is out in 5.3.2.