Add CodeCov

[brunoocasali]

⚠️ This issue is generated, it means the examples and the namings do not necessarily correspond to the language of this repository.
Also, if you are a maintainer, please add any clarification and instructions about this issue.

Sorry if this is already wholly/partially implemented. Feel free to let me know about the state of this issue in the repo.

Related to meilisearch/integration-guides#59

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Codecov helps us to know if a PR add non-tested code. If it's the case, we don't accept the PR.
It's easy to install, easy to configure.

TODO:

• Add CodeCov to this repository.
• Add the CodeCov badge on this repository README.md
• Ensure a minimum coverage is accepted (optional)

[ahmednfwela]

for dart we might be able to use https://pub.dev/packages/coverage
relevant workflow:
https://github.com/dart-lang/coverage/blob/6a59d312874dddb421ae712ffc75e6da9fed0f29/.github/workflows/test-package.yml#L62-L79C43