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>

.eslintrc

@@ -24,6 +24,13 @@ globals:
   SimpleMDE: false
   u2fApi: false
 
+overrides:
+  - files: ["web_src/**/*.worker.js"]
+    env:
+      worker: true
+    rules:
+      no-restricted-globals: [0]
+
 rules:
   arrow-body-style: [0]
   arrow-parens: [2, always]