Private Methods (and accessors) (Stage 3)

Champions: Daniel Ehrenberg @littledan
Spec Repo: https://github.com/littledan/proposal-private-methods
First presented at the July 2017 meeting: https://github.com/tc39/agendas/blob/master/2017/07.md
Moved to Stage 3 at the Sept 2017 meeting: https://docs.google.com/presentation/d/1aI89Jgl7CdtKV6D5-ydieUn_-kgRqAD2X8gGzh62xzc/edit#slide=id.p

Example:

```js
class Counter extends HTMLElement {
#xValue = 0;

get #x() { return this.#xValue; }
set #x(value) {
this.#xValue = value;
window.requestAnimationFrame(
this.#render.bind(this));
}

#clicked() {
this.#x++;
}
}
```

## Implementation

- Initial parser PR (https://github.com/babel/babylon/pull/703) by @Qantas94Heavy

Start with the private methods proposal repository and the initial parser PR linked under Implementation. Check the proposal's current status against this tracking issue and determine whether Babel has an outstanding implementation task. Done means the issue accurately records the remaining work or is updated to reflect the implementation status.

Written by the indexing model from the issue text.