fix(esm): add .js extension to imports

[omichelsen]

This will output ESM valid imports.

However this also breaks unit testing as c8/mocha does not understand .js and is expecting .ts.

Fix: ts-node has experimentalResolver setting to map .js > .ts

[coveralls]

coverage: 100.0%. remained the same
when pulling ac6ff6c on esm-js-extension
into da8ee8a on main.

[pavle-goloskokovic]

This will output ESM valid imports.

However this also breaks unit testing as c8/mocha does not understand .js and is expecting .ts.

Using .js instead of .ts also creates issues when trying to import typescript source files directly into project instead of .js build.

Broken unit tests were a good indicator that this is not the proper way to solve the initial problem.

Hopefully you can find a proper solution that does not require the use of experimental features, and also allow again direct use of typescript sources in projects.