windowrules: add negative: prefix for negating a regex

fixes #8799

src/desktop/Rule.cpp

@@ -0,0 +1,16 @@
+#include "Rule.hpp"
+#include <re2/re2.h>
+
+CRuleRegexContainer::CRuleRegexContainer(const std::string& regex_) {
+    const bool NEGATIVE = regex_.starts_with("negative:");
+
+    negative = NEGATIVE;
+    regex    = std::make_unique<RE2>(NEGATIVE ? regex_.substr(9) : regex_);
+}
+
+bool CRuleRegexContainer::passes(const std::string& str) const {
+    if (!regex)
+        return false;
+
+    return RE2::FullMatch(str, *regex) != negative;
+}