Duplicate notebook menu option (#7374)

* WIP: Create new application:duplicate command, add to menu

* WIP: Duplicate a file, then open the new file.

* Polish

* Removes null check for variable not typed to be null

* Removes unneeded import, type declaration

* Simplifies comments

* Update Playwright Snapshots

* Update Playwright Snapshots

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

packages/application-extension/schema/menus.json

@@ -10,6 +10,10 @@
             "command": "application:rename",
             "rank": 4.5
           },
+          {
+            "command": "application:duplicate",
+            "rank": 4.8
+          },
           {
             "command": "notebook:trust",
             "rank": 20

packages/application-extension/src/index.ts

@@ -83,6 +83,11 @@ const JUPYTERLAB_DOCMANAGER_PLUGIN_ID =
  * The command IDs used by the application plugin.
  */
 namespace CommandIDs {
+  /**
+   * Duplicate the current document and open the new document
+   */
+  export const duplicate = 'application:duplicate';
+
   /**
    * Handle local links
    */
@@ -615,6 +620,20 @@ const title: JupyterFrontEndPlugin<void> = {
         return !!(currentWidget && docManager.contextForWidget(currentWidget));
       };
 
+      commands.addCommand(CommandIDs.duplicate, {
+        label: () => trans.__('Duplicate'),
+        isEnabled,
+        execute: async () => {
+          if (!isEnabled()) {
+            return;
+          }
+
+          // Duplicate the file, and open the new file.
+          const result = await docManager.duplicate(current.context.path);
+          await commands.execute('docmanager:open', { path: result.path });
+        },
+      });
+
       commands.addCommand(CommandIDs.rename, {
         label: () => trans.__('Rename…'),
         isEnabled,