Function createLegacyMessagesBackend

createLegacyMessagesBackend(options: SignerMessagingOptions): MessagingBackend
A MessagingBackend for the legacy direct-message format (NIP-04) over the active account's signer.

Sending encrypts the text to the one recipient with signer().nip04, signs the kind 4 event with the account and publishes it to the recipient's inbox relays and the user's own, so the user's other devices see it too. Receiving subscribes to kind 4 events addressed to or written by the user and decrypts them with the same signer. Conversation ids and message shapes match createPrivateMessagesBackend; groups are refused with invalid:groups-unsupported because the format has none.
Parameters
- options: SignerMessagingOptions
  Store, pool, tiers, and the active account's signer and public key.
Returns MessagingBackend
A backend for the legacy provider; available while the signer has nip04.
Example
```
const backend = createLegacyMessagesBackend({
  store: kernl.eventStore,
  pool: kernl.pool,
  tiers: kernl.relays,
  signer: () => kernl.accounts.active,
  selfPubkey: () => kernl.accounts.active?.pubkey,
});
backend.subscribe({}, (message) => console.log(message.content));
```