Add support for "declare interface" Flow syntax

This has been a feature in Flow for a long time (couldn't easily find a
specific commit adding this). Interfaces are basically undocumented, though, so
it's easy to see how this was missed.

packages/babel-types/src/definitions/flow.js

@@ -66,6 +66,14 @@ defineType("DeclareFunction", {
   }
 });
 
+defineType("DeclareInterface", {
+  visitor: ["id", "typeParameters", "extends", "body"],
+  aliases: ["Flow", "FlowDeclaration", "Statement", "Declaration"],
+  fields: {
+    // todo
+  }
+});
+
 defineType("DeclareModule", {
   visitor: ["id", "body"],
   aliases: ["Flow", "FlowDeclaration", "Statement", "Declaration"],