Add localheinz's PHPStan rules about toolbox

[jakzal]

I'd be careful with adding too many extensions. Currently the build is failing due to one of them: https://travis-ci.com/jakzal/toolbox/jobs/170814684#L959

I think it's best if we support well maintained (preferably official) extensions only, and let people install additional ones in their own images.

[jakzal]

So the question is, is this an extension worth supporting?

[jakzal]

Looks solid, so i'm 👍

[dkarlovi]

Maybe we can figure out a way to separate officially supported from unofficial? Separate namespace and unofficial tags?

The way I see it, having these extensions for PHPStan (and later Psalm, etc) will bring significant additional value to the project since they'll be a valuable part of the toolbox. I see value in having a workflow ready which expects for them to trail for a bit behind the official release, they should be considered "best effort" if they do.

[dkarlovi]

Maybe a namespace can be phpstan-0.10 until it's updated to the latest, then it gets moved to phpstan? Something like that, you can still use it and it might even work (for you), but this way you're communicated to it's not yet up to date.