Add configuration for Psalm

[donnapep]

Is your feature request related to a problem? Please describe

There are gaps in Sensei’s test coverage and some code isn’t easily testable regardless. We would like to gain some quick code quality wins by adopting Psalm, a static code analysis tool.

Describe the solution you'd like

Add a configuration for Psalm such that it uses the least strict level, and only runs checks on code that has changed.

Describe alternatives you've considered

We also considered PHPStan and Phan, but Psalm seems to be used by other Automattic teams more.

Additional context

pdxWSz-15I-p2