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/string-explicit-initialized/input.js

@@ -0,0 +1,4 @@
+enum E of string {
+  A = "a",
+  B = "b",
+}