feat: bal: with --declared, include all declared accounts

[simonmichael]

Together with -E, this allows showing a balance for all accounts, both
used and declared. I mainly want this for hledger-ui, but there's no
harm in exposing it in the balance command as well. This is somewhat
consistent with the accounts and payees commands.

[simonmichael]

I reverted this merged PR after finding issues. I have just pushed a fixed version 6319d61. This was quite tricky; I think it's good now, but I'll wait at least a day, and any feedback is welcome.

Commit message:

feat: bal: with --declared, include declared leaf accounts (#1765)

Together with -E, this shows a balance for both used and declared
accounts (excluding empty parent accounts, which are usually not
wanted in list-mode reports).

This is somewhat consistent with --declared in the accounts and payees
commands, except for the leaf account restriction.

The idea of this is to be able to see a useful "complete" balance
report, even when you don't have transactions in all of your declared
accounts yet. I mainly want this for hledger-ui, but there's no harm
in exposing it in the balance CLI as well.

Doc:

Showing declared accounts

With --declared,
accounts which have been declared with an account directive
will be included in the balance report, even if they have no transactions.
(Since they will have a zero balance, you will also need -E/--empty to see them.)

More precisely, leaf declared accounts (with no subaccounts) will be included,
since those are usually the more useful in reports.

The idea of this is to be able to see a useful "complete" balance report,
even when you don't have transactions in all of your declared accounts yet.

[simonmichael]

Repushed [& merged] with fixes for bs/cf/is, showing the correct declared accounts in each subreport. No known open issues.