windowrules: add negative: prefix for negating a regex

fixes #8799

src/desktop/Rule.hpp

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <memory>
+
+//NOLINTNEXTLINE
+namespace re2 {
+    class RE2;
+};
+
+class CRuleRegexContainer {
+  public:
+    CRuleRegexContainer() = default;
+
+    CRuleRegexContainer(const std::string& regex);
+
+    bool passes(const std::string& str) const;
+
+  private:
+    std::unique_ptr<re2::RE2> regex;
+    bool                      negative = false;
+};