Add Code Coverage report

[andrii-bodnar]

• add CI step that will run tests and generate a code coverage report compatible with the Codecov (it may require some additional configuration. Example: codecov/example-rust)
• add CI step that will publish the report to Codecov (ask @andrii-bodnar to create a repository secret with the token)
• add badge to the Readme

[timofei-iatsenko]

Looks like we need to use nightly Rust to collect coverage 😬
That's a bit weird because release is build with stable rust, and latest nightly has known compatibility problems with SWC.

I will continue investigating.

[dominikdosoudil]

I think that I managed to generate profraw files without nightly rust however I ended at that point because llvm-profdata is probably outdated on my mac 😆. Gonna try some workaround later.