Subsume JSON strings (Stage 4)

By @gibson042, champion: @mathiasbynens

> Presented as Stage 1 at Sept 2017
> Moved to Stage 2 at Nov 2017
> Moved to Stage 3 at Jan 2018
> Moved to Stage 4 at [May 2018](https://github.com/babel/proposals/issues/42#issuecomment-391035527)

Spec repo: https://github.com/tc39/proposal-json-superset

> ECMAScript claims JSON as a subset in JSON.parse, but (as has been well-documented) that is not true because JSON strings can contain unescaped U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR characters while ECMAScript strings cannot.

-[x] Babel PR by @jridgewell: https://github.com/babel/babel/pull/7985

Read the TC39 proposal repository linked in the issue, then inspect Babel PR 7985 by @jridgewell. The issue marks the Babel implementation complete and records the proposal at Stage 4, so the relevant status and existing implementation should be verified before considering any further work.

Written by the indexing model from the issue text.