Enhancement: introduce codecov

[momeemt]

Describe the feature or improvement

The problem is that the compiler is not enough to be tested.
Introducing codecov and visualizing the current progress might motivate us.

https://about.codecov.io/pricing/

Suggested implementation (optional)

none

Component (optional)

If you know which component your issue relates to, please select it:

• CLI tools (brack)
• Tokenizer (brack-tokenizer)
• Parser (brack-parser)
• Transformer (brack-transformer)
• Macro Expander (brack-expander)
• Lower (brack-lower)
• Code Generator (brack-codegen)
• Infrastructure
• Language Server (brack-language-server)
• Project Manager (brack-project-manager)
• GitHub Actions

Additional context

none

Reference (optional)

If this feature request has been discussed or inspired elsewhere, please include links to relevant discussions:

• GitHub Issue (ref: )
• GitHub Pull Request (ref: )
• GitHub Discussion (ref: )
• Discord Server (ref: )

Steps taken to verify

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• Confirmed that a similar feature request does not already exist
• Searched open enhancement issues to avoid duplicates
• Reviewed open enhancement pull requests for possible solutions