Flow enums parsing (#10344)

* Flow enums parsing

* Parse exporting enums

* Enums parsing remove lookahead, other improvements

* Enums: add EnumBody and EnumMember aliases, change boolean members to use BooleaLiteral value

* Fix enum member init flow type, now that boolean members have a BooleanLiteral value

* Flow enums: use contextual utils, change members length checks to use logic operators, remove reserved word logic modification

* Flow enums: remove unnecessary code in generator, fix error message

packages/babel-parser/test/fixtures/flow/enum-declaration/export/input.js

@@ -0,0 +1,3 @@
+export enum A {}
+
+export default enum B {}