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-plugin-transform-flow-comments/test/fixtures/flow-comments/declare-statements/actual.js

@@ -12,3 +12,5 @@ declare class A { static () : number }
 declare class A mixins B<T>, C {}
 declare type A = string
 declare type T<U> = { [k:string]: U }
+declare interface I { foo: string }
+declare interface I<T> { foo: T }

packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/declare-statements/expected.js

@@ -12,3 +12,5 @@
 /*:: declare class A mixins B<T>, C {}*/
 /*:: declare type A = string*/
 /*:: declare type T<U> = { [k:string]: U }*/
+/*:: declare interface I { foo: string }*/
+/*:: declare interface I<T> { foo: T }*/