Implement UnicodeEscape abstract method (#3959)

Also refactored ecma_builtin_json_quote to use the method above

JerryScript-DCO-1.0-Signed-off-by: Adam Szilagyi aszilagy@inf.u-szeged.hu

tests/jerry/es.next/json-stringify.js

@@ -46,3 +46,8 @@ try {
 } catch (e) {
   assert(e instanceof TypeError);
 }
+
+// Checking quoting strings
+assert(JSON.stringify("ab𬄕c") === '"ab𬄕\\u001fc"');
+assert(JSON.stringify("ab\uDC01cd") === '"ab\\udc01c\\u001fd"');
+assert(JSON.stringify("ab\uDC01cd\uD8331e") === '"ab\\udc01c\\u001fd\\ud8331e"');