fix(web): Check the existence of the folder before mkdir.

[bear-shen]

capacitor-filesystem throws error on mkdir when directory already exists
so we need a check before mkdir

otherwise, like this demo

const src = 'http://127.0.0.1:8080/a.mp4';
const tgt = 'video/sample.mp4';

  const fileInfo = await Filesystem.getUri({
    directory: Directory.Documents,
    path: tgt,
  });

  await FileTransfer.downloadFile({
    url: src,
    path: fileInfo.uri,
    progress: true
  });

first time boot, ok
second time, will throw error