ui-extension build copies empty __mocks__ folder to target folder

The ui-extension code contains a folder src/_mocks_ that contains a mock for the penpal library. It's used by Jest when running unit tests.

However, the build system copies this _mocks_ folder to the output path. As a result, the distribution (currently only a Maven JAR file) also contains an empty _mocks_ folder. That should not happen.

Steps to reproduce:

1. cd ui-extension
2. rm -rf target
3. npm ci
4. npm run build

Expected: the folder target/classes/_mocks_ does not exist

Actual:  the folder target/classes/_mocks_ exists