feat: add asdf-gambit

[joeljuca]

Summary

Description:

• Tool repo URL: https://github.com/gambit/gambit
• Plugin repo URL: https://github.com/joeljuca/asdf-gambit/

Checklist

• Format with scripts/format.bash (I moved this file into a Makefile; still it works perfectly)
• 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

[joeljuca]

@jthegedus good evening!

I'm pinging you here in case you haven't seem this one. It's a small PR intended to add a plugin URL for Gambit Scheme. I'm available in case you need to make questions or any additional work on the plugin repo.