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/src/tokenizer/index.js

@@ -1069,6 +1069,7 @@ export default class Tokenizer extends LocationParser {
   readString(quote: number): void {
     let out = "",
       chunkStart = ++this.state.pos;
+    const hasJsonStrings = this.hasPlugin("jsonStrings");
     for (;;) {
       if (this.state.pos >= this.input.length) {
         this.raise(this.state.start, "Unterminated string constant");
@@ -1080,10 +1081,14 @@ export default class Tokenizer extends LocationParser {
         // $FlowFixMe
         out += this.readEscapedChar(false);
         chunkStart = this.state.pos;
+      } else if (
+        hasJsonStrings &&
+        (ch === charCodes.lineSeparator || ch === charCodes.paragraphSeparator)
+      ) {
+        ++this.state.pos;
+      } else if (isNewLine(ch)) {
+        this.raise(this.state.start, "Unterminated string constant");
       } else {
-        if (isNewLine(ch)) {
-          this.raise(this.state.start, "Unterminated string constant");
-        }
         ++this.state.pos;
       }
     }