playground: add a rename button

[ookiineko]

i am not sure if this is the correct way to implement it (im not familiar with web development...), but this seems to work

[netlify[bot]]

✅ Deploy Preview for ffmpegwasm ready!

Name	Link
🔨 Latest commit	706d90a6941de6b76f5d041b971a453725625559
🔍 Latest deploy log	https://app.netlify.com/sites/ffmpegwasm/deploys/659e4c51f353370008f9df57
😎 Deploy Preview	https://deploy-preview-669--ffmpegwasm.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[ghost]

force pushed to fix a bug where the contents of the dialog's input field weren't cleared when the dialog was closed

[jeromewu]

Thanks for the contribution!