Add support for saving file when under a sub-directory

Author: martinRenou

src/drive.ts

@@ -177,7 +177,13 @@ export class FileSystemDrive implements Contents.IDrive {
       throw new Error('No root file handle');
     }
 
-    const handle = await root.getFileHandle(path);
+    let parentHandle = root;
+    // If saving a file that is not under root, we need the right directory handle
+    for (const subPath of path.split('/').slice(0, -1)) {
+      parentHandle = await parentHandle.getDirectoryHandle(subPath);
+    }
+
+    const handle = await parentHandle.getFileHandle(PathExt.basename(path));
     const writable = await handle.createWritable({});
 
     const format = options?.format;
@@ -196,18 +202,14 @@ export class FileSystemDrive implements Contents.IDrive {
     throw new Error('Method not implemented.');
   }
 
-  async createCheckpoint(
-    path: string
-  ): Promise<Contents.ICheckpointModel> {
+  async createCheckpoint(path: string): Promise<Contents.ICheckpointModel> {
     return {
       id: 'test',
       last_modified: new Date().toISOString()
     };
   }
 
-  async listCheckpoints(
-    path: string
-  ): Promise<Contents.ICheckpointModel[]> {
+  async listCheckpoints(path: string): Promise<Contents.ICheckpointModel[]> {
     return [
       {
         id: 'test',