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/test/fixtures/json-strings/string-paragraph-separator/exec.js

@@ -0,0 +1,27 @@
+expect("\u2029".length).toBe(1);
+expect("\u2029").toBe("\u2029");
+
+
+expect("before
after".length).toBe(12);
+expect("before
after").toBe("before\u2029after");
+//            ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char (between 'before' and 'after')
+
+expect("
".length).toBe(1);
+expect("
").toBe("\u2029");
+//      ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\
".length).toBe(0);
+expect("\
").toBe("");
+//       ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\\
".length).toBe(2);
+expect("\\
").toBe("\\\u2029");
+//        ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\\\
".length).toBe(1);
+expect("\\\
").toBe("\\");
+//         ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\\\\
".length).toBe(3);
+expect("\\\\
").toBe("\\\\\u2029");
+//          ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char

packages/babel-plugin-proposal-json-strings/test/fixtures/json-strings/string-paragraph-separator/input.js

@@ -0,0 +1,27 @@
+expect("\u2029".length).toBe(1);
+expect("\u2029").toBe("\u2029");
+
+
+expect("before
after".length).toBe(12);
+expect("before
after").toBe("before\u2029after");
+//            ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char (between 'before' and 'after')
+
+expect("
".length).toBe(1);
+expect("
").toBe("\u2029");
+//      ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\
".length).toBe(0);
+expect("\
").toBe("");
+//       ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\\
".length).toBe(2);
+expect("\\
").toBe("\\\u2029");
+//        ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\\\
".length).toBe(1);
+expect("\\\
").toBe("\\");
+//         ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\\\\
".length).toBe(3);
+expect("\\\\
").toBe("\\\\\u2029");
+//          ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char

packages/babel-plugin-proposal-json-strings/test/fixtures/json-strings/string-paragraph-separator/output.js

@@ -0,0 +1,19 @@
+expect("\u2029".length).toBe(1);
+expect("\u2029").toBe("\u2029");
+expect("before\u2029after".length).toBe(12);
+expect("before\u2029after").toBe("before\u2029after"); //            ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char (between 'before' and 'after')
+
+expect("\u2029".length).toBe(1);
+expect("\u2029").toBe("\u2029"); //      ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\
".length).toBe(0);
+expect("\
").toBe(""); //       ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\\\u2029".length).toBe(2);
+expect("\\\u2029").toBe("\\\u2029"); //        ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\\\
".length).toBe(1);
+expect("\\\
").toBe("\\"); //         ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char
+
+expect("\\\\\u2029".length).toBe(3);
+expect("\\\\\u2029").toBe("\\\\\u2029"); //          ^ That's a U+2029 PARAGRAPH SEPARATOR UTF-16 char