Util: Migrate func/string.php::StrToArray()

To: Util\StringUtil::toArray()
Util: Migrate func/string.php::StrUcfirst2Underline() and reverse

Reverse function is StrUnderline2Ucfirst().

To:

- Util\StringUtil::toCamelCase()
- Util\StringUtil::toSnakeCase()
- Util\StringUtil::toStudlyCaps()
Util: Migrate func/string.php::SubstrIgnHtml()

To: Util\StringUtil::substrIgnHtml()