feat: add Psalm integration

[superrosko]

Q	A
QA	yes

Description

• Create a psalm.xml.dist in the project root
• Copy and paste the contents from this psalm.xml.dist and fix projectFiles
• Run $ composer --dev require vimeo/psalm
• Run $ composer require --dev psalm/plugin-phpunit
• Run $ vendor/bin/psalm --config=psalm.xml.dist --set-baseline=psalm-baseline.xml
• Add a composer script static-analysis with the command psalm --shepherd --stats
• Remove phpstan from the project (phpstan.neon.dist, .travis.yml entry, composer.json require-dev and scripts)

Closes #6