LibWeb: Remove Bindings/Forward.h from LibWeb/Forward.h

This was resulting in a whole lot of rebuilding whenever a new IDL
interface was added.

Instead, just directly include the prototype in every C++ file which
needs it. While we only really need a forward declaration in each cpp
file; including the full prototype header (which itself only includes
LibJS/Object.h, which is already transitively brought in by
PlatformObject) - it seems like a small price to pay compared to what
feels like a full rebuild of LibWeb whenever a new IDL file is added.

Given all of these includes are only needed for the ::initialize
method, there is probably a smart way of avoiding this problem
altogether. I've considered both using some macro trickery or generating
these functions somehow instead.

Userland/Libraries/LibWeb/HTML/HTMLCanvasElement.cpp

@@ -11,6 +11,7 @@
 #include <LibGfx/ImageFormats/JPEGWriter.h>
 #include <LibGfx/ImageFormats/PNGWriter.h>
 #include <LibWeb/Bindings/ExceptionOrUtils.h>
+#include <LibWeb/Bindings/HTMLCanvasElementPrototype.h>
 #include <LibWeb/CSS/StyleComputer.h>
 #include <LibWeb/CSS/StyleValues/IdentifierStyleValue.h>
 #include <LibWeb/CSS/StyleValues/RatioStyleValue.h>