Desktop: Fixes #9543: Fix nested tables not preserved in rich text editor

[personalizedrefrigerator]

Summary

Note

Because this was a feature advertised in the release notes, this feature targets the release-2.13 branch.

The turndown.keep function is only applied after all normal rules (added with .add) don't match.

As such, filter for the table rule needed to be modified to return false in the cases where we want to preserve tables.

Fixes #9543.

Testing

1. Create a note in the rich text editor
2. Add a table (2x3)
3. Add a table within that table
4. Add text within the subtable
5. Switch to the markdown editor and verify that both tables are preserved

Tested on Ubuntu 23.10.

[laurent22]

Thanks for the fix!