Add a setting to prevent sharing projects in public channels (#41395)

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.

crates/worktree/src/worktree_settings.rs

@@ -11,6 +11,8 @@ use util::{
 #[derive(Clone, PartialEq, Eq)]
 pub struct WorktreeSettings {
     pub project_name: Option<String>,
+    /// Whether to prevent this project from being shared in public channels.
+    pub prevent_sharing_in_public_channels: bool,
     pub file_scan_inclusions: PathMatcher,
     pub file_scan_exclusions: PathMatcher,
     pub private_files: PathMatcher,
@@ -51,6 +53,7 @@ impl Settings for WorktreeSettings {
 
         Self {
             project_name: worktree.project_name.into_inner(),
+            prevent_sharing_in_public_channels: worktree.prevent_sharing_in_public_channels,
             file_scan_exclusions: path_matchers(file_scan_exclusions, "file_scan_exclusions")
                 .log_err()
                 .unwrap_or_default(),