feat: add regal plugins

[czchen]

Summary

Description:

• Tool repo URL: https://github.com/StyraInc/regal
• Plugin repo URL: https://github.com/asdf-community/asdf-regal

Checklist

• Format with scripts/format.bash
• Test locally with scripts/test_plugin.bash --file plugins/<your_new_plugin_name>
• Your plugin CI tests are green
  • Tip: use the plugin_test action from asdf-actions in your plugin CI

[czchen]

There is no plugin CI badge SVG anymore. Might need to update CI to remove the test?

[janpieper]

There is no plugin CI badge SVG anymore. Might need to update CI to remove the test?

I just created #943, because I also stumbled upon this issue.

[jthegedus]

Thanks!