Added tests

[slavafomin]

This work is based on the previous PRs #8 and #9 and should be easier to review after their merge.

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This PR introduces the following:

• proper testing infrastructure using Jest with typescript support
• test stubs for all library functions (we just need to implement the tests)
• I've ported old tests and implemented tests for some functions
• I've done some trivial refactoring and improved error handling
• introduced tests coverage support
• implemented codecov integration to display tests coverage
• implemented CI pipeline using GitHub Actions with multibuild (matrix) support for backed node versions and test results display