Convenience method for recursive directory iteration

Migrated from WICG/file-system-access#173 (see #2)

It should be easy to read only files or subfolders from a parent folder, and have a recursive option to return the contents of subfolders. Example:

try {
 for await(let file_reference of folder_reference.read({ files: true, folders: false, recursive: true })) {
  console.log(file_reference)
 }
} catch(error) {
 console.error(error)
}