Set composer package type to wordpress-plugin

[mklepaczewski]

This is standard for WP plugins installed via composer. It allows installation in the correct plugins directory with the following changes to user's project composer.json:

"extra": {
        "installer-paths": {
            "wp-content/mu-plugins/{$name}/": ["type:wordpress-muplugin"],
            "wp-content/plugins/{$name}/": ["type:wordpress-plugin"]
        }
    }

The specific path might vary from project to project. roots/bedrock might use web/app/plugins/{$name}/ instead

[crstauf]

Thanks, @mklepaczewski!