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/.npmignore

@@ -0,0 +1,3 @@
+src
+test
+*.log

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

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+# @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"]
+});
+```

packages/babel-plugin-syntax-json-strings/package.json

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+{
+  "name": "@babel/plugin-syntax-json-strings",
+  "version": "7.0.0-beta.47",
+  "description": "Allow parsing of the U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR in JS strings",
+  "repository": "https://github.com/babel/babel/tree/master/packages/babel-plugin-syntax-json-strings",
+  "license": "MIT",
+  "main": "lib/index.js",
+  "keywords": [
+    "babel-plugin"
+  ],
+  "dependencies": {
+    "@babel/helper-plugin-utils": "7.0.0-beta.47"
+  },
+  "peerDependencies": {
+    "@babel/core": "7.0.0-beta.47"
+  },
+  "devDependencies": {
+    "@babel/core": "7.0.0-beta.47"
+  }
+}

packages/babel-plugin-syntax-json-strings/src/index.js

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+import { declare } from "@babel/helper-plugin-utils";
+
+export default declare(api => {
+  api.assertVersion(7);
+
+  return {
+    manipulateOptions(opts, parserOpts) {
+      parserOpts.plugins.push("jsonStrings");
+    },
+  };
+});