refac: Signal.{notify,abort}를 동기함수로 변경

- async일 필요가 없음

@sammo/util/src/signal.ts

@@ -74,7 +74,7 @@ export class Signal<T = undefined> {
         return result;
     }
 
-    public async abort(): Promise<void> {
+    public abort(): void {
         if(this.resolved){
             return;
         }
@@ -87,7 +87,7 @@ export class Signal<T = undefined> {
         this._resolve({type: 'abort'});
     }
 
-    public async notify(value?: T): Promise<void> {
+    public notify(value?: T): void {
         if(this.resolved){
             return;
         }