Upload coverage data to coveralls/codecov

[ranocha]

Codecov on forks is broken though at the Codecov service level, so we may just need to globally move everything to coveralls.

Mentioned by @ChrisRackauckas in SciML/RootedTrees.jl#170 (comment)

[ChrisRackauckas]

@thazhemadam what would you recommend here?

[thazhemadam]

I think for starters, having codecov run on PRs made directly from within the repos (i.e., not from forks) and on the default branch should prove to be sufficient. I've noticed that the latter doesn't work as intended.
I haven't looked much into Coveralls or their open source plans to comment on that just yet.

[ranocha]

Using Codecov at least for pushes to the main branch would already be nice and improve the status quo. Everything in addition would be very nice to have.

Re Coveralls: You have deleted some working Coveralls code from RootedTrees.jl. It is a bit tricky to get correct coverage with parallel builds, but once everything is set up, it woks well. So far, I have not encountered issues with PRs from forks etc.