add: DevBox Tools - Regex Tester & Timestamp Converter

[yadelishka]

Hello! This PR adds two browser-based developer tools from DevBox Tools:

• Regex Tester — test and debug JavaScript regular expressions in real-time with visual match highlighting and flag support
• Timestamp Converter — convert between Unix timestamps, ISO 8601, RFC 2822, SQL datetime and custom formats with automatic format detection

Both tools are browser-based, 100% free, require no signup or installation, in compliance with the contribution guidelines.

[github-actions]

Hey yadelishka! Thanks for your submission! 🚀

To keep awesome-devtools instantly accessible for everyone, all tools must be:

• Browser-based — no downloads or native app installs
• Direct link — goes straight to the tool (not a multi-tool landing page)
• 100% free — no signups, trials, paywalls, or usage limits

Please make sure your PR meets these criteria. It may take some time for your submission to be reviewed — you can occasionally comment with a "bump" to bring more attention to it.

Thanks for contributing! 🙌