build: Enable codecov with basic configuration

[radeksimko]

Codecov would be useful for identifying potential coverage debt, if there is any, introduced per PRs and assist us in deciding whether to add more tests, and/or ask contributors to.

Some other projects like Terraform, HCL and GoReleaser are using it
https://github.com/hashicorp/terraform/blob/master/codecov.yml

[radeksimko]

This was enabled in #242 and codecov is in active use since then.

[ghost]

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