Add strrev() to NonEmptyStringFunctionsReturnTypeExtension

[nsfisis]

Problem

If strrev() takes non-falsy-string or non-empty-string, it should return non-falsy-string or non-empty-string, but it returns just a string.

https://phpstan.org/r/f9b50c8a-1a20-43ff-8715-42f267a28e24

\PHPStan\dumpType(strrev("abc"));
// => string

Solution

Add strrev() to NonEmptyStringFunctionsReturnTypeExtension.