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-proposal-json-strings/src/index.js

@@ -0,0 +1,29 @@
+import { declare } from "@babel/helper-plugin-utils";
+import syntaxJsonStrings from "@babel/plugin-syntax-json-strings";
+
+export default declare(api => {
+  api.assertVersion(7);
+  const regex = /(\\*)([\u2028\u2029])/g;
+  function replace(match, escapes, separator) {
+    // If there's an odd number, that means the separator itself was escaped.
+    // "\X" escapes X.
+    // "\\X" escapes the backslash, so X is unescaped.
+    const isEscaped = escapes.length % 2 === 1;
+    if (isEscaped) return match;
+
+    return `${escapes}\\u${separator.charCodeAt(0).toString(16)}`;
+  }
+
+  return {
+    inherits: syntaxJsonStrings,
+
+    visitor: {
+      "DirectiveLiteral|StringLiteral"({ node }) {
+        const { extra } = node;
+        if (!extra || !extra.raw) return;
+
+        extra.raw = extra.raw.replace(regex, replace);
+      },
+    },
+  };
+});