Use Babel and ES6 in docs JS files (#31607)

* Pass docs js through Babel

* Use ES6 in docs js

* Only run babel on src files

* Allow babel in Hugo

* Update scripts.html

* Inherit from the root .eslintrc.json

* Use `Array.from`

* Drop Babel from docs

* Prefer template

* replace IIFE with arrow functions

Co-authored-by: XhmikosR <xhmikosr@gmail.com>
Co-authored-by: GeoSot <geo.sotis@gmail.com>

site/static/docs/5.1/assets/js/validate-forms.js

@@ -1,20 +1,19 @@
 // Example starter JavaScript for disabling form submissions if there are invalid fields
-(function () {
+(() => {
   'use strict'
 
   // Fetch all the forms we want to apply custom Bootstrap validation styles to
-  var forms = document.querySelectorAll('.needs-validation')
+  const forms = document.querySelectorAll('.needs-validation')
 
   // Loop over them and prevent submission
-  Array.prototype.slice.call(forms)
-    .forEach(function (form) {
-      form.addEventListener('submit', function (event) {
-        if (!form.checkValidity()) {
-          event.preventDefault()
-          event.stopPropagation()
-        }
+  Array.from(forms).forEach(form => {
+    form.addEventListener('submit', event => {
+      if (!form.checkValidity()) {
+        event.preventDefault()
+        event.stopPropagation()
+      }
 
-        form.classList.add('was-validated')
-      }, false)
-    })
+      form.classList.add('was-validated')
+    }, false)
+  })
 })()