feat(img): add ionImgWillLoad event and emit ionImgDidLoad when image is loaded (#18159)

- Adds `ionImgWillLoad` event that emits when the img src is set
- Moves the `ionImgDidLoad` event emit so that it happens when the image actually finishes loading

fixes #17652 closes #18161

core/src/components.d.ts

@@ -1551,10 +1551,14 @@ export namespace Components {
     */
     'onIonError'?: (event: CustomEvent<void>) => void;
     /**
-    * Emitted when the img src has been set
+    * Emitted when the image has finished loading
     */
     'onIonImgDidLoad'?: (event: CustomEvent<void>) => void;
     /**
+    * Emitted when the img src has been set
+    */
+    'onIonImgWillLoad'?: (event: CustomEvent<void>) => void;
+    /**
     * The image URL. This attribute is mandatory for the <img> element.
     */
     'src'?: string;