extractSigners does not work properly with stable cadence

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Problem

When submitting a transaction using stable cadence syntax, the method extractSigners incorrectly flags something as having more signers than it really does

Steps to Reproduce

transaction {
  prepare(acct: auth(Storage, Capabilities) &Account) { 
    // ...
  }
}

Acceptance Criteria

The correct number of signers is pulled for stable cadence syntax

Context

I was trying to ensure my transactions worked on the emerald city script runner and it doesn't support multi-sig transactions. It uses flow-cadut to pull how many signers there are