Numeric Separator (Stage 3)

Champions: Sam Goto (@samuelgoto), Rick Waldron (@rwaldron)
Spec Repo: https://github.com/tc39/proposal-numeric-separator
Moved to Stage 1 at the May 2017 meeting: https://github.com/tc39/agendas/blob/master/2017/05.md
Moved from Stage 1 to Stage 2 at the July 2017 meeting: https://github.com/tc39/agendas/blob/master/2017/07.md
Moved to Stage 3 at the Nov 2017 meeting: https://github.com/tc39/agendas/blob/master/2017/11.md
Update at May 2018 meeting: https://github.com/tc39/agendas/blob/master/2018/05.md

Syntax

```js
let budget = 1_000_000_000_000;
let nibbles = 0b1010_0001_1000_0101;
let message = 0xA0_B0_C0;
```

## Implementation

- https://github.com/babel/babylon/pull/541, https://github.com/babel/babel/pull/5793
- First released in https://github.com/babel/babel/releases/tag/v7.0.0-alpha.11
- Check the [Spec: Numeric Separator](https://github.com/babel/babylon/issues?q=label%3A%22Spec%3A+Numeric+Separator%22+is%3Aclosed) label in Babylon
- https://babeljs.io/7/
- updating to Stage 2 preset in https://github.com/babel/babel/pull/6071

Start with the TC39 Numeric Separator specification repository and review the linked Babylon and Babel pull requests, release note, and proposal-status page. The issue documents implementation history but does not define a remaining change or a clear completion condition, so confirm current scope before proceeding.

Written by the indexing model from the issue text.