LibWeb: Start fleshing out the ReadableStream interface

This is so we can just assume it exists in Fetch APIs (while still
skipping functionality that relies on a full implementation, of
course).

Userland/Libraries/LibWeb/Forward.h

@@ -366,6 +366,14 @@ namespace Web::ResizeObserver {
 class ResizeObserver;
 }
 
+namespace Web::Selection {
+class Selection;
+}
+
+namespace Web::Streams {
+class ReadableStream;
+}
+
 namespace Web::SVG {
 class SVGAnimatedLength;
 class SVGCircleElement;
@@ -384,10 +392,6 @@ class SVGRectElement;
 class SVGSVGElement;
 }
 
-namespace Web::Selection {
-class Selection;
-}
-
 namespace Web::WebSockets {
 class WebSocket;
 }