Add Angular 9 support

[bsautel]

Does not work with Angular 9:

Compiling ng2-go-top-button : main as commonjs
ERROR in getInternalNameOfClass() called on a non-ES5 class: expected GoTopButton to have an inner class declaration

Would it be compatible to make a new version compatible with Angular 9?

Thanks a lot!

[bsautel]

I created pull request #33 that adds Angular 9 support.

@annapogorelova could you have a look at it please?

[annapogorelova]

Hi, @bsautel, sorry for the late reply and thank you for your work. I have created a new NPM package ng-go-top-button which corresponds to the new Angular library format and supports Angular 9.