Add code coverage bot 🤖

[LucasSte]

Coverage reports will be stored here.

According to Codecov's documentation,

Once merged to the default branch, subsequent pull requests will have checks and report comment.

This is an example of how comments look like: solana-labs/solana#33363 (comment)

[seanyoung]

This does not give a good representation of our coverage; the integration tests aren't run under llvm-cov. For example it looks like the language server is entirely without test coverage; there are no rust tests for the language server, since there is no rust client for a language server (it's tested in integration/vscode/).

[xermicus]

Meanwhile we bumped this to ci-6, don't forget to update the image tag before merging

[LucasSte]

That has not been merged.

[codecov]

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 ☂️

[seanyoung]

Great, fantastic feature for the project

[seanyoung]

any reason if: always() has been removed?

The idea was that if clippy/rustfmt fails, then we can still try to build/test it (is this useful?)

[LucasSte]

That was an oversight. I'll add that again.

[xermicus]

Cool, this is great to have, thanks!