Use customElements polyfill to add Safari support

Due to Safari not supporting customized builtins (is=""
attribute) (https://bugs.webkit.org/show_bug.cgi?id=182671)
use a polyfill to provide this functionality in Safari.

Author: chaimann

admin/app/javascript/solidus_admin/application.js

@@ -1,3 +1,4 @@
 import "@hotwired/turbo-rails"
+import "vendor/custom_elements"
 import "solidus_admin/controllers"
 import "solidus_admin/web_components/solidus_select"

admin/app/javascript/vendor/custom_elements.js

@@ -11,6 +11,8 @@
 
 pin "solidus_admin/application", preload: true
 pin "solidus_admin/utils"
+pin "vendor/custom_elements", preload: true
+
 pin_all_from SolidusAdmin::Engine.root.join("app/javascript/solidus_admin/controllers"), under: "solidus_admin/controllers"
 pin_all_from SolidusAdmin::Engine.root.join("app/components")
 pin_all_from SolidusAdmin::Engine.root.join("app/javascript/solidus_admin/web_components"), under: "solidus_admin/web_components"