Add Psalm support as an alternative or addition to PHPStan

[dantleech]

Psalm is an ever more popular alternative to PHPStan, and is developed full-time at Vimeo.

Features in addition to PHPStan:

• Supports annotating pure functions (i.e. disallow side-effects)
• Finding unused code
• Support for fixing some code automatically.
• ... add more here

Major libraries have are using Psalm-prefixed annotations, including PHPUnit,

It would be nice to have an option to enable it in addition to or as a replacement to PHPStan in the harness.

[tkotosz]

@dantleech what do you think about having both by default? (I don't see why not have both static analyser at the same time, unless they can collide somehow, but I think psalm uses psalm-specific annotations so it should be fine)