Refactor event system to support returnable events

Added support for validating and managing return values in events. Implemented `validate` and `clone` methods in the event base class for event mutation and return handling. Additionally, enhanced error handling, introduced "once" listeners, and improved async execution management in the `EventManager`.

app/src/core/events/Event.ts

@@ -1,17 +1,31 @@
-export abstract class Event<Params = any> {
+export abstract class Event<Params = any, Returning = void> {
+   _returning!: Returning;
+
    /**
     * Unique event slug
     * Must be static, because registering events is done by class
     */
    static slug: string = "untitled-event";
    params: Params;
+   returned: boolean = false;
+
+   validate(value: Returning): Event<Params, Returning> | void {}
+
+   protected clone<This extends Event<Params, Returning> = Event<Params, Returning>>(
+      this: This,
+      params: Params
+   ): This {
+      const cloned = new (this.constructor as any)(params);
+      cloned.returned = true;
+      return cloned as This;
+   }
 
    constructor(params: Params) {
       this.params = params;
    }
 }
 
-// @todo: current workaround: potentially there is none and that's the way
+// @todo: current workaround: potentially there is "none" and that's the way
 export class NoParamEvent extends Event<null> {
    static override slug: string = "noparam-event";
 
@@ -19,3 +33,9 @@ export class NoParamEvent extends Event<null> {
       super(null);
    }
 }
+
+export class InvalidEventReturn extends Error {
+   constructor(expected: string, given: string) {
+      super(`Expected "${expected}", got "${given}"`);
+   }
+}