feat: add nerves-toolchain plugin

[bjyoungblood]

Summary

Description:

• Tool repo URL: https://github.com/nerves-project/toolchains
• Plugin repo URL: https://github.com/nerves-project/asdf-plugin-nerves-toolchain

Checklist

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