It already exists, but most are server-level Punycode conversions.
This converts Punycode with a web browser using JavaScript + JQuery.
It is converted in real time as you type. Comfortable! 😁
Live: https://punycode.net.eu.org/
Simple Source
(Glitch): https://glitch.com/edit/#!/punycode
(Replit): https://replit.com/@balloonvendor/punycode