texttospeech/quickstart.js change the file to use fs/promises

[andriotisnikos1]

hello! ive noticed that texttospeech/quickstart.js uses fs.writeFile synchronously, and is then promisified with 'util.promisify'. why not just use 'fs/promises' and not just 'fs'? it does the same thing, and the code is more understandable. lmk your opinion google team!