LibWeb: Start fleshing out HTML "image requests" and "image data"

This patch adds HTML::ImageRequest and HTML::DecodedImageData.

The latter had to use a different name than "ImageData", as there is
already an IDL-exposed ImageData class in HTML.

Userland/Libraries/LibWeb/Forward.h

@@ -288,6 +288,7 @@ class ClassicScript;
 class CloseEvent;
 class CustomElementDefinition;
 class CustomElementRegistry;
+class DecodedImageData;
 class DocumentState;
 class DOMParser;
 class DOMStringMap;
@@ -371,6 +372,7 @@ class HTMLUListElement;
 class HTMLUnknownElement;
 class HTMLVideoElement;
 class ImageData;
+class ImageRequest;
 class Location;
 class MediaError;
 class MessageChannel;