As a user I want to use private fields similar as in ECMAScript

[jpilgrim]

The ECMAScript proposal

https://github.com/tc39/proposal-class-fields

introduces fields of classes. Public fields are similar to N4JS (except the missing "public" keyword), private fields use a prefix "#". Accessing private fields can either be done with "this.#x" or simply via "#x".

The proposal is now in stage 3 (candidate) meaning that it is almost done.
It is planned for

• Chrome (https://bugs.chromium.org/p/v8/issues/detail?id=5367)
• Babel (Class Fields (Stage 3) babel/proposals#12)

Since it is mainly syntax, we should adapt to that as well.

[jpilgrim]

Closed as this is a duplicate of #1221