Add combine_consecutive_issets and combine_consecutive_unsets rules

[samuelmaudo]

This PR recovers two rules to combine consecutive issets and unsets.

With these rules, this code:

$a = isset($foo) && isset($bar);

unset($foo);
unset($bar);

becomes:

$a = isset($foo, $bar;

unset($foo, $bar);

They were included in the PHP CS Fixer ruleset, so if we replace it by the Symfony ruleset, it might be interesting to include them in our ruleset.