fix(editor): keyboard shortcuts in table cells (#15067)

## Description
Fixes keyboard shortcuts for text formatting (Ctrl+B, Ctrl+I, Ctrl+U,
etc.) not working inside table cells.
## Changes
- **Modified `table-cell.ts`**: Updated the `_handleKeyDown` method to
only prevent default behavior for Tab key and allow other keyboard
events to propagate, enabling text formatting shortcuts to work properly
- **Created `table-keymap.ts`**: New module that registers the
`textKeymap` for table blocks, ensuring text formatting shortcuts are
available in table cells
- **Updated `view.ts`**: Registered the `TableKeymapExtension` in the
table view extension setup
- **Cleaned up `format.ts`**: Removed unnecessary `TextSelection` check
that was preventing shortcuts from working in table contexts
## Closes
Closes #13916 #12127

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## Summary by CodeRabbit

* **Bug Fixes**
* Improved Tab key handling within table cells for more consistent
keyboard navigation.
* Simplified read-only detection for keyboard shortcuts to avoid
unexpected behavior.

* **Refactor**
* Reworked table keyboard mapping and registration to streamline
shortcut handling and event flow.
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