docs(README): use an array of plugins in the example config

[wahidrizka]

Closes #224.

postcss-loader marks the {Object} form of plugins as deprecated ("will be removed in the next major release" in its README, for both webpack.config.js and postcss.config.js), and the config in the Examples section, which the webpack example loads, still used it. As suggested in #224, it now uses an {Array}:

plugins: [
  require('postcss-import')(),
  require('postcss-nested')(),
  ctx.env === 'production' ? require('cssnano')() : false
]

The plugins are required rather than listed by name (as in postcss-loader's own examples), because postcss-load-config rejects names in an {Array} (Invalid PostCSS Plugin found at: plugins[0]). postcss-load-config and postcss-loader both accept this form, and both skip false. A note next to the {Object} form mentions the deprecation.

I ran the old and the new config in development and production through postcss-load-config 6.0.1 and main (on a CSS file with @import and nesting, and on SugarSS with the README's { parser: true, map: 'inline' } context) and through webpack 5.111.1 with postcss-loader 8.2.1 (on the CSS file). The output is identical in all 10 runs, and a control config without postcss-nested differs in all of them.

[ai]

Thanks