Add all BENNC message types with unit tests

src/common.ts

@@ -1,6 +1,12 @@
 export const MAX_DATA_LENGTH = 1000
 
-export enum ByteLength {
-  UInt16 = 2,
-  UInt32 = 4
+export const DEFAULT_KEY = Buffer.alloc(16)
+
+export enum MessageTypes {
+  Subscribe = 0x0000,
+  Basic = 0x0001,
+  UserDataRequest = 0x0002,
+  UserDataResponse = 0x0003,
+  Keepalive = 0x0005,
+  Unsubscribe = 0xFFFF
 }