Add support for default account

[tbm]

In ledger, the default account can be used in the same way as bucket:

account Assets:Cash
    default

2024-07-04 * Foo
    Expenses:Food         10.00 EUR

Result:

          -10.00 EUR  Assets:Cash
           10.00 EUR  Expenses:Food

[tbm]

https://ledger-cli.org/doc/ledger3.html#Command-Directives

The default directive specifies that this account should be used as the “balancing account” for any future transactions that contain only a single posting.