Add a polyfill for String.startsWith, which is missing in some older browsers.
Add a polyfill for String.startsWith, which is missing in some older
browsers.

pranjaltale16/codeworld,venkat24/codeworld,stefan-j/codeworld,parvmor/codeworld,alphalambda/codeworld,stefan-j/codeworld,parvmor/codeworld,pranjaltale16/codeworld,stefan-j/codeworld,nomeata/codeworld,pranjaltale16/codeworld,tgdavies/codeworld,google/codeworld,venkat24/codeworld,alphalambda/codeworld,tgdavies/codeworld,three/codeworld,venkat24/codeworld,google/codeworld,nomeata/codeworld,three/codeworld,google/codeworld,parvmor/codeworld,nomeata/codeworld,three/codeworld,tgdavies/codeworld,alphalambda/codeworld