fix(docs-infra): fix elements example when used with ES5 (#36536)

Previously, the `elements` docs example only worked in browsers that
natively supported Custom Elements and ES2015 modules. Furthermore, it
didn't work on StackBlitz, because StackBlitz ignores the specified
`target` in `tsconfig.json` and uses the UMD bundles (i.e. ES5 code)
even on browsers that do support ES2015.
(NOTE: In the past, this was not a problem, because we explicitly did
not provide a StackBlitz example. This has changed in #36067.)

This commit ensures the example works on all browsers and also on
StackBlitz by providing the necessary Custom Elements polyfills.

Fixes #36532

PR Close #36536

aio/tools/example-zipper/customizer/package-json/elements.json

@@ -8,7 +8,8 @@
     { "name": "e2e", "command": "ng e2e" }
   ],
   "dependencies": [
-    "@angular/elements"
+    "@angular/elements",
+    "@webcomponents/custom-elements"
   ],
   "devDependencies": [
     "@angular-devkit/build-angular",