LibWeb: Replace heycam.github.io/webidl URLs with webidl.spec.whatwg.org

Web IDL is now a WHATWG standard and the specification was moved
accordingly: https://twitter.com/annevk/status/1445311275026821120

The old URLs now redirect, but let's use canonical ones.

Userland/Libraries/LibWeb/DOM/ExceptionOr.h

@@ -111,7 +111,7 @@ public:
 
 private:
     Optional<ValueType> m_result;
-    // https://heycam.github.io/webidl/#idl-exceptions
+    // https://webidl.spec.whatwg.org/#idl-exceptions
     Variant<Empty, SimpleException, NonnullRefPtr<DOMException>> m_exception {};
 };