LibWeb: Add CSSConditionRule

https://www.w3.org/TR/css-conditional-3/#the-cssconditionrule-interface

This simply exposes a condition string, which is implemented differently
in each sub-class.

Userland/Libraries/LibWeb/CSS/CSSConditionRule.cpp

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/CSS/CSSConditionRule.h>
+
+namespace Web::CSS {
+
+CSSConditionRule::CSSConditionRule(NonnullRefPtrVector<CSSRule>&& rules)
+    : CSSGroupingRule(move(rules))
+{
+}
+
+CSSConditionRule::~CSSConditionRule()
+{
+}
+
+}