Add phpstan and check composer script

[n-peugnet]

• add phpstan to dev requirements
• add some more symfony checks
• fix .vscode folder

Infos

PHPStan is in my opinion the most important check tool for PHP. It is configured with a level of 5.
I Also added recommended extensions in this pull request, and by the way I removed .vscode folder from the git as it is environment dependent.

Once this is merged, vscode will suggest to install exension. It is best to install them and uninstall the ones you had previously that do the same thing to avoid collisions (using the filter @installed php and most importantly for "phpstan" and "rename symbols").
Also, as I removed launch.json form .vscode you will have to recreate it with this content:

{
    // Use IntelliSense to learn about possible attributes.
    // Hover to view descriptions of existing attributes.
    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Listen for XDebug",
            "type": "php",
            "request": "launch",
            "port": 9005
        }
    ]
}

Part of #17