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>

package-lock.json

@@ -3659,6 +3659,11 @@
         "domelementtype": "1"
       }
     },
+    "domino": {
+      "version": "2.1.4",
+      "resolved": "https://registry.npmjs.org/domino/-/domino-2.1.4.tgz",
+      "integrity": "sha512-l70mlQ7IjPKC8kT7GljQXJZmt5OqFL+RE91ik5y5WWQtsd9wP8R7gpFnNu96fK5MqAAZRXfLLsnzKtkty5fWGQ=="
+    },
     "dompurify": {
       "version": "2.0.8",
       "resolved": "https://registry.npmjs.org/dompurify/-/dompurify-2.0.8.tgz",
@@ -15008,6 +15013,26 @@
         "errno": "~0.1.7"
       }
     },
+    "worker-loader": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/worker-loader/-/worker-loader-2.0.0.tgz",
+      "integrity": "sha512-tnvNp4K3KQOpfRnD20m8xltE3eWh89Ye+5oj7wXEEHKac1P4oZ6p9oTj8/8ExqoSBnk9nu5Pr4nKfQ1hn2APJw==",
+      "requires": {
+        "loader-utils": "^1.0.0",
+        "schema-utils": "^0.4.0"
+      },
+      "dependencies": {
+        "schema-utils": {
+          "version": "0.4.7",
+          "resolved": "https://registry.npmjs.org/schema-utils/-/schema-utils-0.4.7.tgz",
+          "integrity": "sha512-v/iwU6wvwGK8HbU9yi3/nhGzP0yGSuhQMzL6ySiec1FSrZZDkhm4noOSWzrNFo/jEc+SJY6jRTwuwbSXJPDUnQ==",
+          "requires": {
+            "ajv": "^6.1.0",
+            "ajv-keywords": "^3.1.0"
+          }
+        }
+      }
+    },
     "wrap-ansi": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/wrap-ansi/-/wrap-ansi-2.1.0.tgz",