String mutations #37

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global
_Objects/String/contains
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global
_Objects/String/endsWith
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global
_Objects/String/repeat
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global
_Objects/String/startsWith