Add Refund typings for Stripe (#407)

Also, adding `application` to Event.

lib/stripe/all/stripe.rbi

@@ -562,6 +562,9 @@ module Stripe
   class Event < APIResource
     extend T::Sig
 
+    sig { returns(String) }
+    def account; end
+
     sig { returns(String) }
     def type; end
 
@@ -589,6 +592,17 @@ module Stripe
     def last4; end
   end
 
+  class Refund < APIResource
+    sig {returns(T.nilable(String))}
+    def failure_reason; end
+
+    sig {returns(T.nilable(String))}
+    def payment_intent; end
+
+    sig {returns(T.nilable(String))}
+    def status; end
+  end
+
   class StripeError
     extend T::Sig