tests: add istanbul coverage for tests

[jankapunkt]

As mentioned in discussion #2458 further improvement on this library is stalled as there are not enough automated tests to prevent regressions when reviewing PRs.

A foundation to improve on tests is to run tests with code coverage collected in order to detect yet untested parts of the code and improve the overall test quantity (yes, coverage is not a quality indicator...)

You can run the tests with coverage by npm run test:coverage. Once the tests are done, the coverage is a) printed to the terminal and b) saved in various formats in the (git-ignored) coverage folder.

No current tests were rewritten or changed.

An initial run on my local machine:

npm run test:coverage

> sortablejs@1.15.7 test:coverage
> node ./scripts/coverage.js

Coverage data written to /home/jankapunkt/dev/leaonline/libnpm/Sortable/.nyc_output/coverage.json
-------------|---------|----------|---------|---------|----------------------------------------------------------------------------
File         | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                                                          
-------------|---------|----------|---------|---------|----------------------------------------------------------------------------
All files    |    9.38 |     5.31 |   12.86 |    9.47 |                                                                            
 Sortable.js |    9.38 |     5.31 |   12.86 |    9.47 | ...1-2669,2680-2685,2702-2749,2755-2773,2799,2812-2820,2828-3254,3267-3361 
-------------|---------|----------|---------|---------|----------------------------------------------------------------------------

[jankapunkt]

Is the CI broken or something?