make transactionBuilder.inputs public to generate SourceOutputs

[mr-zwets]

For walletConnect signing with SIGHASH_UTXOS, users need access to the transactions' sourceOutputs.

It is easy to generate sourceOutputs with the transactionBuilder.inputs

but transactionBuilder.inputs is currently a private property.

    // Generate source outputs from inputs (for signing with SIGHASH_UTXOS)
    const sourceOutputs = this.inputs.map((input) => {
      const sourceOutput = {
        amount: input.satoshis,
        to: input.unlocker.generateLockingBytecode(),
        token: input.token,
      };

      return cashScriptOutputToLibauthOutput(sourceOutput);
    });

releated to issue https://github.com/CashScript/cashscript/issues/191 "Create a way for the advanced transaction builder to communicate with WalletConnect for signing"