Angular 9 support

[pascalhaakmat]

This pull request enables the treetable to work with Angular 9. Mostly it's package.json updates.

This removes the @Required decorator since it throws a TypeError (cannot redefine property 'tree').

See also #47

[bohoffi]

@pascalhaakmat could you please fix the build to make this PR mergable?

[gaeljw]

Wow, this is great work. It would be awesome to support Angular9 👍

[clement-lpr]

Thanks for the update effort for A9

Can't wait for it to be deployed :)

[Mario0419]

Any update on this?

[pradeepkv-svec]

@pascalhaakmat

• support for angular 9 is great work.

• I am using this code for generating treetable in my application,but iam facing one issue with the fold method from fp-ts library.Currently fold method is deprecated in fp-ts and angular latest versions.

• i am getting below error while using the fold in tree service file and also in treetable component file.can you please help me to resolve this below error on
  
  error in treetable component file