Add source code.

include/StringUtil.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <cstddef>
+#include <string>
+#include <vector>
+
+namespace StringUtil {
+
+std::string lower(const std::string& text);
+std::string trim(const std::string& text);
+bool equalsIgnoreCase(const std::string& a, const std::string& b);
+bool startsWithIgnoreCase(const std::string& text, const std::string& prefix);
+std::string join(const std::vector<std::string>& parts, std::size_t from, const std::string& separator);
+
+// Number of characters (UTF-8 code points) in the text.
+std::size_t displayWidth(const std::string& text);
+
+// Collapses line breaks into " / ". If `maxChars` > 0 the text is cut to that
+// many characters; 0 means no limit.
+std::string singleLine(const std::string& text, std::size_t maxChars);
+
+}  // namespace StringUtil