Remove extract-loader

[Mr0grog]

The old extract-loader Webpack plugin is no longer maintained and is the source of all our remaining dependency security warnings. Looking at its issue tracker, it seems like there are also a number of edge-cases where it does not work correctly with Webpack 5 that have been slowly piling up (we haven't hit those yet, but who knows when that might happen). This switches to the currently recommended by Webpack mini-css-extract-plugin.

The main difference here is that this outputs a single bundled CSS file (<root>/bundle.css) instead of converting the individual files by themselves. I think that’s probably OK.

An alternative would be to have postcss-loader write the files out itself. That gets us something more like the old file structure, but feels like it’s cutting against the grain a bit. It also seems like sourcemaps get lost when I do that, although I’m sure that, with enough fiddling, I could figure out how to put them back. For future reference, that webpack config would look like:

      {
        test: /\.css$/,
        type: 'asset/resource',
        generator: {
          filename: removeSrcDirectory
        },
        exclude: [
          path.resolve(__dirname, 'node_modules')
        ],
        // Only process legacy CSS files
        include: [
          path.resolve(__dirname, 'src/css/styles.css'),
          path.resolve(__dirname, 'src/css/diff.css')
        ],
        use: [
          // {
          //   loader: 'extract-loader',
          // },
          // {
          //   loader: 'css-loader',
          //   options: {
          //     import: false,
          //     sourceMap: true
          //   },
          // },
          {
            loader: 'postcss-loader',
            options: {
              sourceMap: true,
              postcssOptions: {
                plugins: [
                  autoprefixer()
                ]
              }
            },
          },
        ],
      },