feat: add scaffold from hay-kot/scaffold

[particledecay]

Summary

Description: Add the scaffold plugin.

• Tool repo URL: https://github.com/hay-kot/scaffold
• Plugin repo URL: https://github.com/particledecay/asdf-scaffold

Checklist

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