add declare_strict_types to .php_cs

[SebSept]

As recommended (and that's a good point :) )
https://devdocs.prestashop.com/1.7/development/coding-standards/#strict-typing

But, adding this requires the cs-fixer:init to have the option to chose for another file...
then the command becomes a bit useless since it's action is just to copy a file.

maybe, the strict type could be include in the original file of prestashop/php-dev-tools.
I should submit an issue.

the pros of using the prestashop/php-dev-tools is that I do not have to maintain the file.