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-parser/test/fixtures/experimental/json-strings/string-paragraph-separator/input.js

@@ -0,0 +1,2 @@
+("before
after");
+//      ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char (between 'before' and 'after')