feat(Guild): add fetchBan method (#3726)

* Add error for not resolving ID to fetch ban

* Add Guild#fetchBan

* add missing !

* typings

* lint fixes

* add jsdoc description

src/structures/Guild.js

@@ -554,6 +554,21 @@ class Guild extends Base {
    * @property {?string} reason Reason the user was banned
    */
 
+  /**
+  * Fetches information on a banned user from this guild.
+   * @param {UserResolvable} user The User to fetch the ban info of
+   * @returns {BanInfo}
+   */
+  fetchBan(user) {
+    const id = this.client.users.resolveID(user);
+    if (!id) throw new Error('FETCH_BAN_RESOLVE_ID');
+    return this.client.api.guilds(this.id).bans(id).get()
+      .then(ban => ({
+        reason: ban.reason,
+        user: this.client.users.add(ban.user),
+      }));
+  }
+
   /**
    * Fetches a collection of banned users in this guild.
    * @returns {Promise<Collection<Snowflake, BanInfo>>}