Add code coverage in the CI

[robUx4]

As we are adding more tests, we can start using some code coverage system. That might give us an idea of what part of the code are lacking tests. And maybe notice some dead part of the code that can be removed as well.

This probably requires a lot more tests to have a clearer picture. But that's almost mandatory nowadays for a data parser. We may also notice some bugs before others do...

This doesn't make much sense in libmatroska although it could do with a bit of testing for a the few custom code it has (blocks, cues, etc).

We can probably use codecov which is free for Open Source projects https://github.com/codecov/codecov-action