Internal slot properties (#7947)

* Support internalSlots to babel-types and babel-generator

* Parsing support for internal slot properties

* Print internal slots in babel-generator

* Add whitespace before first internal slot property

packages/babylon/test/fixtures/flow/interfaces-module-and-script/2/output.json

@@ -114,6 +114,7 @@
           "callProperties": [],
           "properties": [],
           "indexers": [],
+          "internalSlots": [],
           "exact": false
         }
       }