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.

Case-insensitivity in `substr_compare` won't work for multibyte strings.
Eg. `substr_compare('строка', 'CТРОКА'...` with `$case_insensitivity = true` won't work.