Adopt ECMAScript Private Fields

[kitsonk]

We should adopt ECMAScript private fields in the internal code within Deno and in std. We have a couple of different patterns in the code base in regards to this (TypeScript private and using WeakMaps in Events) and should adopt a consistent mechanism which provides us with the runtime privacy of data. We have a 3rd pattern where we have marked some fields as private, but they are defined with a symbol to make it easy to utilise an interator interface. We might reconsider dropping the private keyword.

Private fields do not apply to methods (fields are not part of prototype, while methods are), so not all of our use of the private keyword could be easily addressed (I am not too personally keen on turning methods into fields, as having them part of the prototype is far more efficient).

TypeScript 3.8 will support ECMAScript private fields, V8 already supports them. But, right now prettier and dprint do not. We still use prettier for formatting our internal code, and dprint for deno fmt.

Refs #3937
Refs prettier/prettier#7263
Refs dsherret/dprint#91