Subsume json (#7985)

* Add Subsume JSON transform

https://github.com/tc39/proposal-json-superset is at stage 3.

This allows U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR to appear unescaped inside strings and directives.

* Move to Stage 3

* Break diretive parsing

* Update README

* Handle multi-escape sequences

* Remove babylon files after rename

packages/babel-plugin-syntax-json-strings/README.md

@@ -0,0 +1,35 @@
+# @babel/plugin-syntax-json-strings
+
+Allow parsing of the U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR in JS strings
+
+## Installation
+
+```sh
+npm install --save-dev @babel/plugin-syntax-json-strings
+```
+
+## Usage
+
+### Via `.babelrc` (Recommended)
+
+**.babelrc**
+
+```json
+{
+  "plugins": ["@babel/plugin-syntax-json-strings"]
+}
+```
+
+### Via CLI
+
+```sh
+babel --plugins @babel/plugin-syntax-json-strings script.js
+```
+
+### Via Node API
+
+```javascript
+require("@babel/core").transform("code", {
+  plugins: ["@babel/plugin-syntax-json-strings"]
+});
+```