Revert "DEV: Drop old IE11 intersection-observer references" (#13017)

This reverts commit 7360a0f70fae59bb065dffb9b1ad42d13ff38f46.

iOS still wants this sometimes. Probably best to revert for now and we can
always remove this again later.

See: https://meta.discourse.org/t/189799/11?u=blake

lib/tasks/javascript.rake

@@ -140,6 +140,9 @@ def dependencies
       source: 'mousetrap/plugins/global-bind/mousetrap-global-bind.js'
     }, {
       source: 'resumablejs/resumable.js'
+    }, {
+      # TODO: drop when we eventually drop IE11, this will land in iOS in version 13
+      source: 'intersection-observer/intersection-observer.js'
     }, {
       source: 'workbox-sw/build/.',
       destination: 'workbox',