Fix strict type errors detected by phpstan

There are 271 errors identified in the code quality report, or 266 in the test report.

mstrelan → changed the visibility of the branch 11.x to hidden.

Not sure the best way to split this, so just started working on it. We can cherry-pick each fix to separate issues if we need to split it.

I don't think we need checkArgumentsPassedByReference here as that isn't affected by strict types (AFAIK).

Removed the bit about excluding tests. Tests call out to non-test code, and there can exist errors.

I rebased the MR hoping to see the number of errors go down since 📌 Fix more var and param docs identified by phpstan Active , but it's actually gone up from 381 to 436.

@mstrelan: Meh! Are the new additions all in the var/param docs region? Or were there a lot of additions to the baseline during that period?

@spokje I didn't look at what they were. It wouldn't have been baseline additions, because these issues are only reported once the config is changed as per the MR here.