Install phpstan and fix some errors

[mpesari]

Description

Run PHPStan (static analyser) in workflow and fix some errors found by it.

You can run it manually with: vendor/bin/phpstan

Needs rebase after #59 is merged.

[VuorelaJ-P]

I would like to have phpstan in composer scripts as well. Could be as part of composer lint -command, since it mostly is for code quality. But since it has own existing action in github, maybe own command would be better in pipeline sense.

[mpesari]

It's important to run static analysis with different PHP versions to ensure interoperability with promised legacy versions (ie not use too new language features). Not sure if this can be done without using the separate action? I'm not very familiar with GitHub Actions.

Maybe add composer analyse? This way it would still be separate in the pipeline. In our project we also use Psalm which could be added to the same composer script.

[VuorelaJ-P]

Analyze sounds good command.
Actions can have only one command if I recall correctly. But we can always add command like composer check or something that runs composer analyze and composer lint as pre commit hook, when adding those later in line.
But that is more sens to set when code quality is in level that it should be in order to ensuring maintaining it that way. This repo is really old, from Checkout era from different version control, so it still has probably quite few out dated conventions and such.

You can also add psalm in this PR as well, I don't think it breaks too much, and would anyway makes sense to have, since there's already plan to add it anyway when have time to do so.

[mpesari]

Looks like some of the dependencies pulled in by psalm are not compatible with PHP 7.3 :)

I made another branch psalm based on this one, but it can't be merged right now.

[mpesari]

I added the command composer analyze which now just runs phpstan.

Let me know when #59 is merged - I'll rebase afterwards