Add undeclared xhr2 dependency

This used directly in src/app/core/services/server-xhr.service.ts.
We have been relying on this being available by virtue of it being
a transitive dependency of some other package.

Author: alanorth

package-lock.json

@@ -161,6 +161,7 @@
     "reflect-metadata": "^0.2.2",
     "rxjs": "^7.8.0",
     "uuid": "^8.3.2",
+    "xhr2": "^0.2.1",
     "zone.js": "~0.14.10"
   },
   "devDependencies": {