feat(GuildChannel): add manageable getter (#2439)

* Adds GuildChannel.manageable

* Resolve requested changes

* fix eslint max-len error

* Fix for nullable permissionsFor()

* Indent fixes

src/structures/GuildChannel.js

@@ -539,6 +539,17 @@ class GuildChannel extends Channel {
     return this.permissionsFor(this.client.user).has(Permissions.FLAGS.MANAGE_CHANNELS);
   }
 
+  /**
+   * Whether the channel is manageable by the client user
+   * @type {boolean}
+   * @readonly
+   */
+  get manageable() {
+    if (this.client.user.id === this.guild.ownerID) return true;
+    const permissions = this.permissionsFor(this.client.user);
+    return permissions && permissions.has([Permissions.FLAGS.MANAGE_CHANNELS, Permissions.FLAGS.VIEW_CHANNEL]);
+  }
+
   /**
    * Deletes this channel.
    * @param {string} [reason] Reason for deleting this channel