[PECO-1390] Configure Typescript for tests

[kravets-levko]

PECO-1390

• Configured nyc to run tests via ts-node;
• All imports in tests updated to use /lib instead of /dist so nyc can properly collect coverage data. A great side-effect of this - no longer need to build sources before runing tests;
• For now, all tests are still in .js files and still have to use require. Migrating all tests to TS would be a very massive change, so it will be done gradually in follow-up PRs.

After merging this PR, a slight (<1%) difference in code coverage may be caused by testing original sources vs. compiled+source maps, and using ts-node instead of regular node.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.56%. Comparing base (1e4b730) to head (4180896).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #256 +/- ## ========================================== - Coverage 92.68% 92.56% -0.12% ========================================== Files 64 65 +1 Lines 1545 1547 +2 Branches 273 273 ========================================== Hits 1432 1432 - Misses 47 49 +2 Partials 66 66 ```

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