Provide marker conversion handling for table cells

[scofalik]

📝 Provide a description of the new feature

I am not sure if it is a new feature or improvement, but...

In short, it would be nice if table cells behave like images or other widgets when it comes to marker conversion. Right now, they behave like paragraphs, that is, if a marker is created on a cell, the conversion happens for the cell contents not for the cell itself. As a result, the text is highlighted, while it would be better if the cell had received proper classes:

Model now:

[<tableCell><paragraph>Foo</paragraph></tableCell>]

View now:

<td><span class="marker">Foo</span></td>

View expected:

<td class="marker">Foo</td>

This way we can do better styling, for example change table border or background.

This is fairly easy to achieve as proper code already exists.

Of course, this is needed to implement collaboration features for multi-cell selection (comments in this case). However, since downcasting for tables is quite complicated, it cannot be easily done on collaboration features side (it is possible, I did it, but it would require tens of lines of two function calls and it would be limited to comments only).

I will create a branch with a proposal for changes for table package.

Related: #7359

[scofalik]

https://github.com/ckeditor/ckeditor5/compare/i/7360?expand=1