Ignore test items in the npm package

Ignore test items in the npm package

### What?

Add a `.npmignore` file to skip `test` and `testData` folders.

### Why?

Skip unnecessary data from the package. See [npm-developers](https://npmjs.org/doc/developers.html). For this project, that's 64 Ko less.

### Test?

Folders are indeed skipped. Should not change anything in the rest of the lib., test data is not supposed to be needed in prod.