Add stylelint to HMR

[wturnerharris]

Barrel WordPress will include stylelint in an upcoming release.

Since adding this to the local webpack.config.js has no effect on HMR, we need to add the following to the webpack.js routine:

const StyleLintPlugin = require('stylelint-webpack-plugin')
...
  preparePlugins () {
...
      ...(this.watching && config.get('hmr') ? [
        new webpack.HotModuleReplacementPlugin()
      ]: []),
      new StyleLintPlugin({
        configFile: path.resolve(config.get('cwd'), '.stylelintrc'),
        files: '(src/css/|modules/)**/*.css',
        syntax: 'scss',
        failOnError: false,
        quiet: false
      }),  
      new webpack.NoEmitOnErrorsPlugin(),
    ]
  }

These are the deps required:

"stylelint": "^9.6.0",
"stylelint-config-standard": "^18.2.0",
"stylelint-webpack-plugin": "^0.10.5",