LibWeb: Avoid including Navigable.h in headers

This greatly reduces how much is recompiled when changing Navigable.h,
from >1000 to 82.

Libraries/LibWeb/HTML/UserNavigationInvolvement.cpp

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2024, Andrew Kaster <andrew@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/Event.h>
+#include <LibWeb/HTML/UserNavigationInvolvement.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/browsing-the-web.html#event-uni
+UserNavigationInvolvement user_navigation_involvement(DOM::Event const& event)
+{
+    // For convenience at certain call sites, the user navigation involvement for an Event event is defined as follows:
+
+    // 1. Assert: this algorithm is being called as part of an activation behavior definition.
+    // 2. Assert: event's type is "click".
+    VERIFY(event.type() == "click"_fly_string);
+
+    // 3. If event's isTrusted is initialized to true, then return "activation".
+    // 4. Return "none".
+    return event.is_trusted() ? UserNavigationInvolvement::Activation : UserNavigationInvolvement::None;
+}
+
+}