Add the VSCode PHPUnit extension, for simple phpunit access through VSCode

[joshsedl]

Furthermore, while we are at it, we should think about defining some constraints for phpcs in VSCode through:
"phpsab.snifferArguments": [ "--file-list=<fileList>" ],

The defines "[a] file containing a list of files and/or directories to check (one per line)". This way, less problems get displayed on the VSCode "Problems" tab and the phpunit problems would appear more clearly.

[JPustkuchen]

Being able to see code coverage etc. in VSCode would surely be helpful, at least for unit tests! I like the idea, but it's indeed low prio.