Missing "offset" class property in Spreadsheet_Excel_Writer_Worksheet

[ttgrules]

Looks like "offset" was missed as a dynamic property of Spreadsheet_Excel_Writer_Worksheet.

It gets set in Workbook.php around lines 581 and 681, and will throw an error in PHP 8.3.

[sanmai]

Thanks for the PR! If you could find yourself writing a unit test hitting this line, I'd really, really appreciate that! But we'll ship it nevertheless.

[ttgrules]

@sanmai Good call on the unit test! It looks like existing test cases hit it (every time ->close() is called on a workbook), but phpunit is currently configured to ignore deprecation warnings and move on.

Might be a broader question of what you want to catch or ignore, but I put together a pull request to update the PHPUnit config to show Deprecation notices and convert them to exceptions.

#51