chore: upload coverage report using CODECOV_TOKEN

[vil02]

Closes #453. Uses the same logic as in TheAlgorithms/Java#5100:

• the tokenless upload is used for PRs from forks,
• the upload with token is used only for PRs and pushes within this repository.

Similar to TheAlgorithms/Rust#670.

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Comments in areas I changed are up to date
• I have added comments to hard-to-understand areas of my code
• I have made corresponding changes to the README.md

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.84%. Comparing base (c9e2fa1) to head (8e6da79).
Report is 3 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@ ## master #454 +/- ## ========================================== - Coverage 94.94% 94.84% -0.10%  ========================================== Files 234 234 Lines 9843 9895 +52 Branches 1382 1393 +11 ========================================== + Hits 9345 9385 +40  - Misses 384 392 +8  - Partials 114 118 +4 

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[siriak]

Thanks a lot!