fix(core): mark zone.js as an optional peer dependency (angular#61616)

`zone.js` is no longer mandatory.

PR Close angular#61616

Author: alan-agius4

packages/core/package.json

@@ -26,6 +26,9 @@
   "peerDependenciesMeta": {
     "@angular/compiler": {
       "optional": true
+    },
+    "zone.js": {
+      "optional": true
     }
   },
   "repository": {