Added startsWith() and endsWith() for yii\helpers\StringHelper

See #5316

I've tested different implementations, big strings and `mb_substr` + `mb_strlen` with `8bit` check is pretty efficient.
Case-insensitive check of course is much slower, where additional `mb_strtolower()` is triggered to ignore the case for multibyte strings.

That will require additional length check of initial string (which could be large), because of :
http://php.net/manual/en/function.substr-compare.php#110859
> If offset is equal to or greater than the length of main_str, or the length is set and is less than 1 (prior to PHP 5.6), substr_compare() prints a warning and returns FALSE.