LibWeb: Add ProcessingInstruction node

Not particuarly useful right now, but is used in
ensure_pre_insertion_validity.

I'm not totally sure on the constructor arguments.

Userland/Libraries/LibWeb/Forward.h

@@ -60,6 +60,7 @@ class MouseEvent;
 class Node;
 class ParentNode;
 class Position;
+class ProcessingInstruction;
 class ShadowRoot;
 class Text;
 class Timer;
@@ -300,6 +301,7 @@ class MouseEventWrapper;
 class NodeWrapper;
 class PerformanceTimingWrapper;
 class PerformanceWrapper;
+class ProcessingInstructionWrapper;
 class ProgressEventWrapper;
 class ScreenWrapper;
 class ScriptExecutionContext;