feat(StageChannel): add createStageInstance method & use better naming convention (#5951)

Co-authored-by: SpaceEEC <spaceeec@yahoo.com>

src/structures/StageChannel.js

@@ -20,14 +20,23 @@ class StageChannel extends BaseGuildVoiceChannel {
   }
 
   /**
-   * The instance of this stage channel, if it exists
+   * The stage instance of this stage channel, if it exists
    * @type {?StageInstance}
    * @readonly
    */
-  get instance() {
+  get stageInstance() {
     return this.guild.stageInstances.cache.find(stageInstance => stageInstance.channelID === this.id) ?? null;
   }
 
+  /**
+   * Creates a stage instance associated to this stage channel.
+   * @param {StageInstanceCreateOptions} options The options to create the stage instance
+   * @returns {Promise<StageInstance>}
+   */
+  createStageInstance(options) {
+    return this.guild.stageInstances.create(this.id, options);
+  }
+
   /**
    * Sets the RTC region of the channel.
    * @name StageChannel#setRTCRegion

src/structures/StageInstance.js

@@ -60,7 +60,7 @@ class StageInstance extends Base {
   }
 
   /**
-   * The stage channel associated with this instance
+   * The stage channel associated with this stage instance
    * @type {?StageChannel}
    * @readonly
    */