Change build (DSP-790)

[tobiasschweizer]

resolves DSP-790

This PR changes the lib's build options so it is built as an es6 module (before "commonjs" was used). This is necessary because Angular 10 shows warnings for libs built with "commonjs".

• mocha with esm: https://stackoverflow.com/questions/46487307/is-it-possible-to-use-es6-modules-in-mocha-tests/46487904
• mocha declarations: https://stackoverflow.com/questions/39816482/how-to-import-describe-and-it-from-mocha-in-typescript