Attempt to add the IntersectionObserver polyfill

Why:

* The site is not working in Safari, missing the IntersectionObserver
  polyfill, which we are apparently not importing correctly.

This change addresses the issue by:

* Implementing the configuration suggested in
  gatsbyjs/gatsby#10435 (comment)

gatsby-browser.js

@@ -4,6 +4,8 @@
  * See: https://www.gatsbyjs.org/docs/browser-apis/
  */
 
-exports.onInitialClientRender = () => {
-  require("intersection-observer")
+export const onClientEntry = async () => {
+  if (typeof IntersectionObserver === "undefined") {
+    await import("intersection-observer")
+  }
 }