LibWeb: Make :hover invalidation logic reusable for all pseudo classes

We achieve this by keeping track of all checked pseudo class selectors
in the SelectorEngine code. We also give StyleComputer per-pseudo-class
rule caches.

Libraries/LibWeb/CSS/PseudoClassBitmap.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2025, Andreas Kling <andreas@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/PseudoClass.h>
+
+namespace Web::CSS {
+
+class PseudoClassBitmap {
+public:
+    PseudoClassBitmap() = default;
+    ~PseudoClassBitmap() = default;
+
+    void set(PseudoClass pseudo_class, bool bit)
+    {
+        size_t const index = to_underlying(pseudo_class);
+        if (bit)
+            m_bits |= 1LLU << index;
+        else
+            m_bits &= ~(1LLU << index);
+    }
+
+    bool get(PseudoClass pseudo_class) const
+    {
+        size_t const index = to_underlying(pseudo_class);
+        return (m_bits & (1LLU << index)) != 0;
+    }
+
+    void operator|=(PseudoClassBitmap const& other)
+    {
+        m_bits |= other.m_bits;
+    }
+
+private:
+    u64 m_bits { 0 };
+};
+
+// NOTE: If this changes, we'll have to tweak PseudoClassBitmap a little bit :)
+static_assert(to_underlying(PseudoClass::__Count) <= 64);
+
+}