ARC-60 (algorandfoundation/ARCs#494) adds an optional StdSigData.scheme:
omitted means ed25519, and falcon-1024-det signs with an algorand/falcon
deterministic key. createStdSignData no longer throws 4200 for
post-quantum accounts:

- A Falcon-1024 account (pqsig scheme "f1") gets signer = pqsig.pk and
  scheme = "falcon-1024-det".
- The pqsig must still authorize the account (addressFromPQSig equals
  authAddr ?? address, from algod); otherwise it throws 4603
  (ERROR_INVALID_SIGNER), e.g. after a rekey since connecting.
- Any other post-quantum scheme throws 4601 (ERROR_UNSUPPORTED_SCHEME)
  before querying algod.
- ed25519 requests are unchanged and omit scheme, so wallets that
  predate it keep working.

Adds getSignDataScheme(emptySignature), exported from
@txnlab/use-wallet and /adapter, which returns the ARC-60 scheme for an
account ('ed25519', 'falcon-1024-det' or undefined), and the
SignDataScheme type. Siwa.type widens to SignDataScheme, since wallets
require a SIWA payload's type to match the scheme. StdSignData gains
scheme?: string.