Add phpstan extensions

[msvrtan]

In order for phpstan to be even more helpful, lets install some extensions:

• phpstan/phpstan-doctrine
• phpstan/phpstan-symfony
• phpstan/phpstan-webmozart-assert
• phpstan/phpstan-phpunit

This PR includes commits from #175 & #176 so please check just the last changes

[HcsOmot]

UPDATE: The container file is there, but it's named with an upper case letter A, instead of a lowercase, like phpstan documentation example states.

~Still don't know what AppDevDebugProjectContainer is and what it's used for - don't know what to google for.~

Apparently, it's the config file for symfony's DIC. I just assumed it was, when I commented yesterday, but wasn't sure.

~Looks like SF Extension doesn't evaluate the path to services container. Is there something more we can do to confirm this and report an issue?~

I don't have any experience with this, so don't really know how to make sure this is correct and not make a fool of myself.

[msvrtan]

Wow, many thanks @HcsOmot !