LibC: Implement CODESET for langinfo

Userland/Libraries/LibC/langinfo.cpp

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <langinfo.h>
+
+static const char* __nl_langinfo(nl_item item)
+{
+    switch (item) {
+    case CODESET:
+        return "UTF-8";
+    default:
+        return "";
+    }
+}
+
+extern "C" {
+
+char* nl_langinfo(nl_item item)
+{
+    // POSIX states that returned strings should not be modified,
+    // so this cast is probably fine.
+    return const_cast<char*>(__nl_langinfo(item));
+}
+}