This PR adds a setting to prevent projects from being shared in public
channels.

This can be enabled by adding the following to the project settings
(`.zed/settings.json`):

```json
{
  "prevent_sharing_in_public_channels": true
}
```

This will then disable the "Share" button when not in a private channel:

<img width="380" height="115" alt="Screenshot 2025-10-28 at 2 28 10 PM"
src="https://github.com/user-attachments/assets/6761ac34-c0d5-4451-a443-adf7a1c42bcd"
/>

Release Notes:

- collaboration: Added a `prevent_sharing_in_public_channels` project
setting for preventing projects from being shared in public channels.