Add code coverage

[ntadej]

Add code coverage with reporting to codecov.

An additional target called coverage is available which also generates HTML for local inspection.

[codecov-commenter]

Welcome to Codecov 🎉

Once merged to your default branch, Codecov will compare your coverage reports and display the results in this comment.

Thanks for integrating Codecov - We've got you covered ☂️

[HarelM]

I don't have good knowledge of how to generate the lcov file, and I would start with simply uploading the coverage report and later on define failure threshold, but overall this looks good.

[ntadej]

I just copied and adapted the config from the main MapLibre Native. We can also ignore or revise the test later if failing too much.

[ntadej]

For reference, this is the report: https://app.codecov.io/gh/maplibre/maplibre-native-qt/tree/coverage