docs: es6ify 'Breaking Changes' and 'File' API pages (#15507)

* docs: apply arrow functions to app.makeSingleInstance example

* docs: apply arrow functions to session docs

* docs: change normal to arrow functions in File object docs

docs/api/file-object.md

@@ -15,15 +15,15 @@ Example of getting a real path from a dragged-onto-the-app file:
 </div>
 
 <script>
-  document.addEventListener('drop', function (e) {
+  document.addEventListener('drop', (e) => {
     e.preventDefault();
     e.stopPropagation();
-    
-    for (let f of e.dataTransfer.files) {
+
+    for (const f of e.dataTransfer.files) {
       console.log('File(s) you dragged here: ', f.path)
     }
   });
-  document.addEventListener('dragover', function (e) {
+  document.addEventListener('dragover', (e) => {
     e.preventDefault();
     e.stopPropagation();
   });