fix: apply message size limit before decoding message

If we apply the message size limit after decoding the message it's
too late as we've already processed the bad message.

Instead, if the buffer full of unprocessed messages grows to be
large than the max message size (e.g. we have not recieved a complete
message under the size limit), throw an error which will cause the
stream to be reset.

src/decode.ts

@@ -3,6 +3,8 @@ import { Uint8ArrayList } from 'uint8arraylist'
 import type { Source } from 'it-stream-types'
 import type { Message } from './message-types.js'
 
+export const MAX_MSG_SIZE = 1 << 20 // 1MB
+
 interface MessageHeader {
   id: number
   type: keyof typeof MessageTypeNames