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>

web_src/js/features/highlight.worker.js

@@ -0,0 +1,12 @@
+import {highlightBlock} from 'highlight.js';
+import {createWindow} from 'domino';
+
+self.onmessage = function ({data}) {
+  const window = createWindow();
+  self.document = window.document;
+
+  const {index, html} = data;
+  document.body.innerHTML = html;
+  highlightBlock(document.body.firstChild);
+  self.postMessage({index, html: document.body.innerHTML});
+};