add utf8.escape, string.escape & reformat extensions.md

src/util/stringutil.cpp

@@ -7,8 +7,7 @@
 #include <sstream>
 #include <stdexcept>
 
-// TODO: finish
-std::string util::escape(const std::string& s) {
+std::string util::escape(std::string_view s) {
     std::stringstream ss;
     ss << '"';
     size_t pos = 0;