Add code coverage reporting for project.

[teymour-aldridge]

Describe the feature you'd like
Code coverage reporting (percentage of lines of code which are run during Yew's tests).

Is your feature request related to a problem? Please describe. (Optional)
This should be possible to do using grcov (but I need to check if it works with Wasm)

Describe alternatives you've considered (Optional)
No code coverage.

Additional context (Optional)
Add any other context or screenshots about the feature request here.

Questionnaire

• I'm interested in implementing this myself but don't know where to start
• I would like to add this feature
• I don't have time to add this right now, but maybe later

[jstarry]

This would be really nice!

[teymour-aldridge]

I imagine we'd use a service like codecov.io to handle the reporting of test results.