feat: Implement creating split transactions.

[bvanelli]

Here is one example:

import datetime
from actual import Actual
from actual.queries import create_transaction, create_splits, get_or_create_account

with Actual(
    base_url="http://localhost:5006", password="mypass", encryption_password="mypass", file="My budget"
) as actual:
    today = datetime.date.today()
    act = get_or_create_account(actual.session, "My account")
    t = create_transaction(actual.session, today, act, "My payee", notes="My first transaction", amount=-10.5)
    taxes = create_transaction(actual.session, today, act, "Government", "Damn taxes", category="Taxes", amount=-3.0)
    create_splits(actual.session, (t, taxes))
    actual.commit()

and the result on the server:

Closes #24

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.30508% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.54%. Comparing base (f7d81a1) to head (fde4258).

Files	Patch %	Lines
actual/queries.py	97.87%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #26 +/- ## ========================================== + Coverage 96.16% 96.54% +0.37% ========================================== Files 9 9 Lines 1434 1447 +13 ========================================== + Hits 1379 1397 +18 + Misses 55 50 -5 ```

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