Add support in the ci for phpstan

Descriptive title for this enhancement

we need to add static code analysis to our ci with phpstan

Actual behavior

we only run php lint et code sniffer on our code actually

Expected behavior

find error by running code analysis on our code

Step by step description of new behaviour

1. copy the .neon file from https://gitlab.fusiondirectory.org/fusiondirectory/dev-tools/-/tree/master/phpstan/fusiondirectory/1.4-dev into the root of the core source
2. run phpstan -c phpstan.neon

Benefits

get static analysis of the code

Possible Drawbacks

none

Applicable Issues