Combine postcss with icon font generation warning

[Wolfr]

It is currently not possible to combine postcss as an css compiler together with generating an icon font.

  css: {
    compiler: 'scss',
    minify: false,
    purge: false
  },

  icons: {
    generateIconFont: false,
    iconFontPath: "./content/scss/_icon-font.scss",
    svgIconClassPrefix: 'o-svg-icon',
    iconFontClassPrefix: 'if'
  },

Bedrock should throw an error if you attempt to do so.

[Wolfr]

This relates to https://github.com/usebedrock/bedrock/pull/403 - as it's about similar settings.

However as I don't know anyone actually using postcss in combination with an icon font, I will make this a low prio item.