chat: add is_member field to Chat

chat.go

@@ -37,6 +37,10 @@ type Chat struct {
 	LastName  string `json:"last_name"`
 	Username  string `json:"username"`
 
+	// Still shows whether the user is a member
+	// of the chat at the moment of the request.
+	Still bool `json:"is_member,omitempty"`
+
 	// Returns only in getChat
 	Photo            *ChatPhoto `json:"photo,omitempty"`
 	Description      string     `json:"description,omitempty"`