Add default notebook feature and delete action for Zed editor

[Pujan-Dev]

Objective

Improve notebook cell deletion to prevent accidental data loss by requiring confirmation before a cell is deleted. This PR also enables the notebooks feature flag for all users by default.

Solution

• Added a new DeleteCurrentCell action that prompts the user for confirmation before deleting the currently selected notebook cell.
• Updated the notebook UI delete button and tooltip to use DeleteCurrentCell instead of directly deleting the cell.
• Added the required prompt and error-handling support for the confirmation flow.
• Enabled the notebooks feature flag for all users by having NotebookFeatureFlag::enabled_for_all() return true.
• Made minor code organization and action-handling improvements related to notebook cell deletion.

Testing

• Tested the notebook cell deletion flow and verified that selecting delete displays a confirmation prompt.
• Verified that the cell is deleted only after the user confirms.
• Verified that cancelling the prompt leaves the notebook cell unchanged.
• Verified that the notebook feature is available without user-specific feature flag configuration.
• Additional testing of notebook cell deletion across supported platforms would be useful.

Self-Review Checklist:

• [ x ] I've reviewed my own diff for quality, security, and reliability
• [ x ] Unsafe blocks (if any) have justifying comments
• [ x ] The content adheres to Zed's UI standards ([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist) and [icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md) guidelines)
• [ x ] Tests cover the new/changed behavior
• [ x ] Performance impact has been considered and is acceptable

Showcase

Click to view showcase

When deleting a notebook cell, Zed now displays a confirmation prompt before performing the deletion. Confirming removes the selected cell, while cancelling preserves it.

this image have the delete button logo on the side as per the zed's ui.

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this issue was raised on the
Release Notes:

• Improved: Added a confirmation prompt when deleting notebook cells to help prevent accidental data loss.
• Improved: Enabled the notebooks feature for all users by default.