LibWeb: Add the ability for an AbortSignal to follow another

Following another abort signal basically means to make an abort signal
abort when another abort signal is aborted, unless the following signal
is already aborted.

Userland/Libraries/LibWeb/DOM/AbortSignal.h

@@ -38,6 +38,8 @@ public:
 
     JS::ThrowCompletionOr<void> throw_if_aborted() const;
 
+    void follow(JS::NonnullGCPtr<AbortSignal> parent_signal);
+
 private:
     explicit AbortSignal(JS::Realm&);