Add NIP-26 delegation token signing function

[joshr4]

Exposing the nostr-tools createDelegation function so that the extension can sign a delegation token using its saved private key. The parameters arg is expected to have this type https://github.com/nbd-wtf/nostr-tools/blob/master/nip26.ts#L8

type Parameters = {
  pubkey: string // the key to whom the delegation will be given
  kind: number | undefined
  until: number | undefined // delegation will only be valid until this date
  since: number | undefined // delegation will be valid from this date on
}

[githubbbie]

Could you rework this? thx