feat(compatibility): add polyfill.io to improve compatibility (#236)

assets/lib/polyfill.yml

@@ -6,3 +6,7 @@ algoliasearch:
   - Promise
   - Object.entries
   - Object.assign
+theme:
+  - Object.values
+  - Promise
+  - fetch

layouts/partials/assets.html

@@ -255,6 +255,9 @@
     window.config = {{ jsonify $config | safeJS }};
 </script>
 
+{{- /* Polyfill.io */ -}}
+{{- partial "plugin/script.html" (dict "source" "https://polyfill.io/v3/polyfill.min.js?features=Element.prototype.closest%2CrequestAnimationFrame%2CCustomEvent%2CPromise%2CObject.entries%2CObject.assign%2CObject.values%2Cfetch") -}}
+
 {{- range $scratch.Get "script" -}}
     {{- partial "plugin/script.html" . -}}
 {{- end -}}