LibWeb: Begin implementing the SVGLength type

There are a few unimplemented features for this type:

1. The value setter should throw a DOMException if it is invoked on an
   SVGLength that was declared readonly in another IDL file.

2. SVG::AttributeParser does not parse unit types when it parses lengths
   so all SVGLength will have an "unknown" unit for now.

3. Due to (2), methods which convert between units are unimplemented.

Userland/Libraries/LibWeb/SVG/SVGLength.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2022, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <AK/Types.h>
+#include <AK/Weakable.h>
+#include <LibWeb/Bindings/Wrappable.h>
+#include <LibWeb/DOM/ExceptionOr.h>
+
+namespace Web::SVG {
+
+// https://www.w3.org/TR/SVG11/types.html#InterfaceSVGLength
+class SVGLength
+    : public RefCounted<SVGLength>
+    , public Bindings::Wrappable
+    , public Weakable<SVGLength> {
+public:
+    using WrapperType = Bindings::SVGLengthWrapper;
+
+    static NonnullRefPtr<SVGLength> create(u8 unit_type, float value);
+    virtual ~SVGLength() = default;
+
+    u8 unit_type() const { return m_unit_type; }
+
+    float value() const { return m_value; }
+    DOM::ExceptionOr<void> set_value(float value);
+
+private:
+    SVGLength(u8 unit_type, float value);
+
+    u8 m_unit_type { 0 };
+    float m_value { 0 };
+};
+
+}