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/HTMLModElement.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/Assertions.h>
+#include <LibWeb/Bindings/HTMLModElementPrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/HTML/HTMLModElement.h>