fix(positioning): prevent memory leak in positioning service

PositioningService is listening to events in the constructor but it
never got cleaned up on destroy. Since we can have many instances of
that service, it could pile up and use more and more memory.

Author: AurelienRichez

src/positioning/positioning.service.ts

@@ -1,12 +1,12 @@
 import { Injectable, ElementRef, RendererFactory2, Inject, PLATFORM_ID, NgZone } from '@angular/core';
+import { takeUntilDestroyed } from '@angular/core/rxjs-interop';
 import { isPlatformBrowser } from '@angular/common';
 
 import { positionElements } from './ng-positioning';
 
 import { fromEvent, merge, of, animationFrameScheduler, Subject, Observable } from 'rxjs';
 import { Options } from './models';
 
-
 export interface PositioningOptions {
   /** The DOM element, ElementRef, or a selector string of an element which will be moved */
   element?: HTMLElement | ElementRef | string;
@@ -64,7 +64,7 @@ export class PositioningService {
           this.update$$
         );
 
-        this.triggerEvent$.subscribe(() => {
+        this.triggerEvent$.pipe(takeUntilDestroyed()).subscribe(() => {
           if (this.isDisabled) {
             return;
           }