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-line-separator/input.js

@@ -0,0 +1,2 @@
+("before
after");
+//      ^ That's a U+2028 LINE SEPARATOR UTF-16 char (between 'before' and 'after')

packages/babel-parser/test/fixtures/experimental/json-strings/string-line-separator/output.json

@@ -0,0 +1,107 @@
+{
+  "type": "File",
+  "start": 0,
+  "end": 101,
+  "loc": {
+    "start": {
+      "line": 1,
+      "column": 0
+    },
+    "end": {
+      "line": 2,
+      "column": 83
+    }
+  },
+  "program": {
+    "type": "Program",
+    "start": 0,
+    "end": 101,
+    "loc": {
+      "start": {
+        "line": 1,
+        "column": 0
+      },
+      "end": {
+        "line": 2,
+        "column": 83
+      }
+    },
+    "sourceType": "script",
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start": 0,
+        "end": 17,
+        "loc": {
+          "start": {
+            "line": 1,
+            "column": 0
+          },
+          "end": {
+            "line": 1,
+            "column": 17
+          }
+        },
+        "expression": {
+          "type": "StringLiteral",
+          "start": 1,
+          "end": 15,
+          "loc": {
+            "start": {
+              "line": 1,
+              "column": 1
+            },
+            "end": {
+              "line": 1,
+              "column": 15
+            }
+          },
+          "extra": {
+            "rawValue": "before
after",
+            "raw": "\"before
after\"",
+            "parenthesized": true,
+            "parenStart": 0
+          },
+          "value": "before
after"
+        },
+        "trailingComments": [
+          {
+            "type": "CommentLine",
+            "value": "      ^ That's a U+2028 LINE SEPARATOR UTF-16 char (between 'before' and 'after')",
+            "start": 18,
+            "end": 101,
+            "loc": {
+              "start": {
+                "line": 2,
+                "column": 0
+              },
+              "end": {
+                "line": 2,
+                "column": 83
+              }
+            }
+          }
+        ]
+      }
+    ],
+    "directives": []
+  },
+  "comments": [
+    {
+      "type": "CommentLine",
+      "value": "      ^ That's a U+2028 LINE SEPARATOR UTF-16 char (between 'before' and 'after')",
+      "start": 18,
+      "end": 101,
+      "loc": {
+        "start": {
+          "line": 2,
+          "column": 0
+        },
+        "end": {
+          "line": 2,
+          "column": 83
+        }
+      }
+    }
+  ]
+}