Migrate all TypeScript private properties to JavaScript private properties (#)

[NachoVazquez]

The Problem

Most private properties of the project are using the Typescript private indicator instead of the native ES private properties (#).

ESLint private properties are better for libraries since they actually prohibit consumers from accessing the properties, even at runtime. This prevents unwanted dependencies on library internals.

Acceptance Criteria

Migrate all Typescript private properties to ESLint private properties.

[NachoVazquez]

Block until a new version of eslint-plugin-rxjs-angular is released with the merged fixes

[santoshyadavdev]

Hey @NachoVazquez,
Let me know if this is ready for development.

[NachoVazquez]

Sadly, this PR hasn't been merged yet cartant/eslint-plugin-rxjs-angular#12.

We have a couple of options here.

1. Drop eslint-plugin-rxjs-angular seems it is unmaintained by now
2. Fork eslint-plugin-rxjs-angular and start maintaining it ourselves
3. Try to reach Cartant and see if he plans to maintain the project

Any thoughts @LayZeeDK ?