Add type: wordpress-plugin to composer.json to help with setting up custom installation paths

[sdunham]

Super tiny PR which adds a type of wordpress-plugin which is one of the several supported WordPress types for the Composer installers package. This allows you to set up custom installer paths for WP plugins like this via your project's composer.json file:

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