Add onHTML.com to Programming Tools

[mimturan]

https://onhtml.com/

onHTML.com is a free online HTML/CSS/JS live editor bundled with 60+ built-in developer tools (JSON formatter, regex tester, JWT decoder, color picker, gradient maker, image compressor, base64 encoder, UUID generator, and more). It works entirely client-side, requires no signup or account, and is open source on GitHub (https://github.com/mimturan/onhtml). It fits well in the Programming Tools section alongside CSS Minifier, JavaScript compressor, Carbon, and similar dev tools.

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• I have read the Contribution guidelines and I am confident that my PR is suitable.
• - [x] This pull request has a descriptive title. For example, Add [App name], not Update readme.md or Added new entries.
• - [x] The app I added is not a duplicate.