Add Codecov action

[gdalle]

I discovered in #40 that some lines were never tested, it would be good to have a Codecov workflow to spot that. With such a small package we should always be at 100% coverage

[ChrisRackauckas]

@thazhemadam is going to do an overhaul centralization of our CI scripts soon, so this will get handled at that time.

[gdalle]

Still it wouldn't cost much to activate it here, since I spotted uncovered lines and am currently doing a rather big overhaul in #40

[gdalle]

I can do the yml part myself but I cannot add codecov as a GitHub app to the repo cause I'm not part of SciML

[ChrisRackauckas]

All repos already have codecov.

[gdalle]

Cool, I'll add the line to the workflow

[gdalle]

Can I ask you for a look at #40 if you have some time? It's very minorly breaking but it will be a huge help for getting sparsity to work across backends