feat: add guild.fetchVanityCode() (#2732)

* Error for guild.fetchVanityURL feature not enabled

* added GET/guilds/{guild.id}/vanity-url endpoint

* fix: code conventions

* adopted suggestion

* Changed error message according to change request

* Renamed method

to indicate that only the code is fetched, not the entire url.

* Update Guild.js

src/structures/Guild.js

@@ -434,6 +434,26 @@ class Guild extends Base {
       });
   }
 
+  /**
+   * Fetches the vanity url invite code to this guild.
+   * Resolves with a string matching the vanity url invite code, not the full url.
+   * @returns {Promise<string>}
+   * @example
+   * // Fetch invites
+   * guild.fetchVanityCode()
+   *   .then(code => {
+   *     console.log(`Vanity URL: https://discord.gg/${code}`);
+   *   })
+   *   .catch(console.error);
+   */
+  fetchVanityCode() {
+    if (!this.features.includes('VANITY_URL')) {
+      return Promise.reject(new Error('VANITY_URL'));
+    }
+    return this.client.api.guilds(this.id, 'vanity-url').get()
+      .then(res => res.code);
+  }
+
   /**
    * Fetches all webhooks for the guild.
    * @returns {Promise<Collection<Snowflake, Webhook>>}