Make it possible to serialize decimal from json_arrays

[kod-kristoff]

@pytest.mark.parametrize(
    "json_format", [json_arrays.JsonFormat.JSON, json_arrays.JsonFormat.JSON_LINES]
)
def test_dump_decimal(
    json_format: json_arrays.JsonFormat, snapshot, array_of_dicts: list[dict]
) -> None:
    with tempfile.TemporaryFile() as fp:
        json_arrays.dump([10.20, "10.20", Decimal("10.20")], fp, json_format=json_format)
        fp.seek(0)
        data_written = fp.read()

    assert data_written == snapshot