Upgrade PHPStan on the PHP harness

[dantleech]

Currently we are on 0.12 but PHPStan is now on 1.6.

Some upgrade notes:

In VP Sylius (i guess for Symfony haress) we have the following config:

        "phpstan/extension-installer": "^1.1",
        "phpstan/phpstan": "^1.0",
        "phpstan/phpstan-doctrine": "1.0",
        "phpstan/phpstan-phpunit": "^1.0",
        "phpstan/phpstan-symfony": "^1.0",
        "phpstan/phpstan-webmozart-assert": "^1.0",

    "config": {
        "allow-plugins": {
            //...
            "phpstan/extension-installer": true,

notes:

• the phpstan-baseline.neon is included automatically now.
• the "extension installer" is a composer plugin which automatically registers the phpstan extensions