Disallow offset access on string

[mvorisek]

I propose to add, to strict rules, a rule to reject string offset access like:

$str[$index];
$str[5];

in favor of

substring($str, $index, 1)
substring($str, 5, 1)

The main motivation is to detect wrongly declared types. Currently when $var is declared wrongly as string instead of string[], it is undetected.