imp: add: Verify balance assertions on each posting (#2355)

[reesmichael1]

As requested in #2355, this verifies balance assertions during hledger add. Here's an example of the new workflow. With the following journal file:

2025-03-18 * Deposit
    Assets:Checking                     $20
    Income:Salary                      $-20

$ hledger add
Date [2025-03-19]:
Description: Food
Account 1: Expenses:Food
Amount  1: $15
Account 2: Assets:Checking
Amount  2 [$-15]: $-15 == $10
1:6:
  | 2025-03-19 Food
  |     Expenses:Food               $15
3 |     Assets:Checking            $-15 == $10
  |                                     ^^^^^^
  |                                  $0

Balance assertion failed in Assets:Checking
Across all commodities at this point, excluding subaccounts, ignoring costs,
the asserted balance is:        $10
but the calculated balance is:   $5
(difference: $5)
To troubleshoot, check this account's running balance with assertions disabled, eg:
hledger reg -I 'Assets:Checking$'
Amount  2 [$-15]: $-15  == $5
Account 3 (or . or enter to finish this transaction):
2025-03-19 Food
    Expenses:Food               $15
    Assets:Checking            $-15 == $5

Save this transaction to the journal ? [y]: y
Saved.

The only thing that might be odd here is including the "To troubleshoot..." lines. I left those in because it's not bad advice for tracking down why an assertion might be failing, but also it isn't really helpful and might be misleading in the middle of an hledger add, so I'm open to removing those lines here.