code coverage should go back to original ts code

[jspaaks]

see also #15, #6

[jspaaks]

stacktraces should also give pointers to original code

[sverhoeven]

Tested with class with 2 functions and testing one of them. Sadly reported coverage is incorrect.

[sverhoeven]

See if https://github.com/kulshekhar/ts-jest works.
Seems it doesn't transpile ts>babel, but only ts, which might also work.

[sverhoeven]

The coverage can be generated with

CI=1 npm test -- --coverage --no-cache

The coverage can be found at coverage/remapped/html/index.html.

Not sure if transpile pipeline ts>babel>js gets correct coverage.
My tests in https://github.com/3D-e-Chem/molviewer-tsx report the correct coverage.