LibWeb: Abstract Range's members into AbstractRange

Range's member variables are stored in AbstractRange as per the spec,
as they are also shared with StaticRange.

Userland/Libraries/LibWeb/DOM/AbstractRange.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2022, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <LibWeb/Bindings/Wrappable.h>
+
+namespace Web::DOM {
+
+class AbstractRange
+    : public RefCounted<AbstractRange>
+    , public Bindings::Wrappable {
+public:
+    using WrapperType = Bindings::AbstractRangeWrapper;
+
+    virtual ~AbstractRange() override = default;
+
+    Node* start_container() { return m_start_container; }
+    const Node* start_container() const { return m_start_container; }
+    unsigned start_offset() const { return m_start_offset; }
+
+    Node* end_container() { return m_end_container; }
+    const Node* end_container() const { return m_end_container; }
+    unsigned end_offset() const { return m_end_offset; }
+
+    // https://dom.spec.whatwg.org/#range-collapsed
+    bool collapsed() const
+    {
+        // A range is collapsed if its start node is its end node and its start offset is its end offset.
+        return start_container() == end_container() && start_offset() == end_offset();
+    }
+
+protected:
+    AbstractRange(Node& start_container, u32 start_offset, Node& end_container, u32 end_offset)
+        : m_start_container(start_container)
+        , m_start_offset(start_offset)
+        , m_end_container(end_container)
+        , m_end_offset(end_offset)
+    {
+    }
+
+    NonnullRefPtr<Node> m_start_container;
+    u32 m_start_offset;
+
+    NonnullRefPtr<Node> m_end_container;
+    u32 m_end_offset;
+};
+
+}