Implement a custom navigation through tables

[Reinmar]

Currently, we allow the browser to do its thing. Some support Tab, most support arrow keys, but all that is inconsistent and buggy. We should design and implement our own solution.

[Reinmar]

In fact, this is a special case of https://github.com/ckeditor/ckeditor5-widget/issues/84.

I think that the widget feature should introduce a base way to navigate to nested editables and then the table feature should override that in a way which will make sense for tables.

[Reinmar]

Two stages:

• Handling selection at the cell boundary. When the selection is at the start and you press up, then you go to the end of the cell above the current cell. When you press left, you go to the end of the cell on the left. Symmetrical for the end of cell positions. Note: an empty table creates an interesting case and should be handled correctly.
• Handling selections not at the cell boundary by comparing rects.

In both cases, let's try to mimic Google Docs behavior as closely as possible, with the exception that when you move to the adjacent cell, move to the end/start of it.

[oleq]

Some tickets to review/close along with this one:

• A way to leave a table with keyboard #3286
• Should Shift-TAB on selected table go to the last table cell #3177
• The tab key should leave the editor #1709