LibWeb/CSS: Reimplement var()/attr() as arbitrary substitution functions

"Arbitrary substitution functions" are a family of functions that
includes var() and attr(). All of them resolve to an arbitrary set of
component values that are not known at parse-time, so they have to be
substituted at computed-value time.

Besides it being nice to follow the spec closely, this means we'll be
able to implement the others (such as `if()` and `inherit()`) more
easily.

The main omission here is the new "spread syntax", which can be
implemented in the future.

Libraries/LibWeb/CSS/Parser/ArbitrarySubstitutionFunctions.h

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/Parser/Parser.h>
+
+namespace Web::CSS::Parser {
+
+// https://drafts.csswg.org/css-values-5/#substitution-context
+struct SubstitutionContext {
+    enum class DependencyType : u8 {
+        Property,
+        Attribute,
+        Function,
+    };
+    DependencyType dependency_type;
+    String first;
+    Optional<String> second {};
+
+    bool is_cyclic { false };
+
+    bool operator==(SubstitutionContext const&) const;
+    String to_string() const;
+};
+
+class GuardedSubstitutionContexts {
+public:
+    void guard(SubstitutionContext&);
+    void unguard(SubstitutionContext const&);
+
+private:
+    Vector<SubstitutionContext&> m_contexts;
+};
+
+enum class ArbitrarySubstitutionFunction : u8 {
+    Attr,
+    Var,
+};
+[[nodiscard]] Optional<ArbitrarySubstitutionFunction> to_arbitrary_substitution_function(FlyString const& name);
+
+bool contains_guaranteed_invalid_value(Vector<ComponentValue> const&);
+
+[[nodiscard]] Vector<ComponentValue> substitute_arbitrary_substitution_functions(DOM::AbstractElement&, GuardedSubstitutionContexts&, Vector<ComponentValue> const&, Optional<SubstitutionContext> = {});
+
+using ArbitrarySubstitutionFunctionArguments = Vector<Vector<ComponentValue>>;
+[[nodiscard]] Optional<ArbitrarySubstitutionFunctionArguments> parse_according_to_argument_grammar(ArbitrarySubstitutionFunction, Vector<ComponentValue> const&);
+
+[[nodiscard]] Vector<ComponentValue> replace_an_arbitrary_substitution_function(DOM::AbstractElement&, GuardedSubstitutionContexts&, ArbitrarySubstitutionFunction, ArbitrarySubstitutionFunctionArguments const&);
+
+}