Move syntax highlighting to web worker (#11017)

This should eliminate page freezes when loading big files/diff.
`highlightBlock` is needed to preserve existing nodes when highlighting
and for that, highlight.js needs access to the DOM API so I added a DOM
implementation to make it work, which adds around 300kB to the output
file size of the lazy-loaded `highlight.js`.

Co-authored-by: Lauris BH <lauris@nix.lv>

webpack.config.js

@@ -51,6 +51,7 @@ module.exports = {
         sourceMap: true,
         extractComments: false,
         terserOptions: {
+          keep_fnames: /^(HTML|SVG)/, // https://github.com/fgnass/domino/issues/144
           output: {
             comments: false,
           },
@@ -89,6 +90,19 @@ module.exports = {
         test: require.resolve('jquery-datetimepicker'),
         use: 'imports-loader?define=>false,exports=>false',
       },
+      {
+        test: /\.worker\.js$/,
+        use: [
+          {
+            loader: 'worker-loader',
+            options: {
+              name: '[name].js',
+              inline: true,
+              fallback: false,
+            },
+          },
+        ],
+      },
       {
         test: /\.js$/,
         exclude: /node_modules/,