Re-add code coverage

[JeroenDeDauw]

Using codecov.io

Broken ScrutinizerCI coverage disabled with 4369ec7

Can copy from https://github.com/JeroenDeDauw/PsrLogTestDoubles

[reedy]

JeroenDeDauw/PsrLogTestDoubles@840df99 moved to using shepherd after dropping ScrutinizerCI in JeroenDeDauw/PsrLogTestDoubles@f8d8bde

[JeroenDeDauw]

Shepard is for type coverage stats

That repo, and many others I am involved with, use Codecov for code coverage tracking. Codecov changed some stuff again recently, so many of those repos are not a great example. Here is an up to date one, though it's a MW extension rather than a PHP library, so keep that in mind https://github.com/ProfessionalWiki/Rules