LibWeb: Reduce recompilation impact of DOM/Element.h

Remove unused/redundant includes from Element.h:
- AK/IterationDecision.h (redundant)
- ARIA/AttributeNames.h (redundant via ARIAMixin.h)
- CSS/CascadedProperties.h (redundant via PseudoElement.h)
- CSS/StylePropertyMapReadOnly.h (pointer types only)
- HTML/LazyLoadingElement.h (unused in header)

Extract IntersectionObserverRegistration struct from
IntersectionObserver.h into its own lightweight header.
This breaks the heavy transitive include chain through
IntersectionObserverEntry.h and Geometry/DOMRect.h that
was pulled into every file including Element.h.

Indirect recompilation impact reductions:
- IntersectionObserver.h: ~1387 -> ~27 files
- LazyLoadingElement.h: ~1387 -> ~1002 files

Libraries/LibWeb/IntersectionObserver/IntersectionObserverRegistration.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <LibGC/Ptr.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::IntersectionObserver {
+
+// https://www.w3.org/TR/intersection-observer/#intersectionobserverregistration
+struct IntersectionObserverRegistration {
+    // https://www.w3.org/TR/intersection-observer/#dom-intersectionobserverregistration-observer
+    // [A]n observer property holding an IntersectionObserver.
+    GC::Ref<IntersectionObserver> observer;
+
+    // https://www.w3.org/TR/intersection-observer/#dom-intersectionobserverregistration-observer
+    // NOTE: Optional is used in place of the spec using -1 to indicate no previous index.
+    // [A] previousThresholdIndex property holding a number between -1 and the length of the observer's thresholds property (inclusive).
+    Optional<size_t> previous_threshold_index;
+
+    // https://www.w3.org/TR/intersection-observer/#dom-intersectionobserverregistration-previousisintersecting
+    // [A] previousIsIntersecting property holding a boolean.
+    bool previous_is_intersecting { false };
+};
+
+}