Add initial FreeBSD support (#20480)

This PR adds initial support for FreeBSD
(https://github.com/zed-industries/zed/issues/15309). While there is
still work left to be done, it seems to be usable. As discussed by
@syobocat (https://github.com/zed-industries/zed/discussions/10247), the
changes were just adding ```target_os = "freebsd"``` to wherever it
checks if the OS is Linux.


![image](https://github.com/user-attachments/assets/80ea5b29-047f-4cbd-8263-42e5fa6c94b7)

Needs to be build with ```RUSTFLAGS="-C link-dead-code"```

Known Issues:
- There's an issue in ```crates/project/src/environment.rs``` where a
command fails because ```/bin/sh``` on FreeBSD doesn't support the
```-l``` option.

![image](https://github.com/user-attachments/assets/c3c38633-160f-4f47-8840-e3da67f6ebc8)
- The file/folder choosers provided by the ```ashpd``` crate don't work
on FreeBSD (at least with KDE). This isn't that bad since a fallback
dialog is used.

![image](https://github.com/user-attachments/assets/29373006-1eb9-4ed0-bd52-2d0047fab418)
 - Moving to trash won't work.
- Numerous tests fail (when running on FreeBSD). While I haven't looked
into this much, it appears that the corresponding features seem to work
fine.

Release Notes:

- Added initial support for FreeBSD

crates/gpui/src/platform/test/platform.rs

@@ -28,7 +28,7 @@ pub(crate) struct TestPlatform {
     active_display: Rc<dyn PlatformDisplay>,
     active_cursor: Mutex<CursorStyle>,
     current_clipboard_item: Mutex<Option<ClipboardItem>>,
-    #[cfg(target_os = "linux")]
+    #[cfg(any(target_os = "linux", target_os = "freebsd"))]
     current_primary_item: Mutex<Option<ClipboardItem>>,
     pub(crate) prompts: RefCell<TestPrompts>,
     pub opened_url: RefCell<Option<String>>,
@@ -59,7 +59,7 @@ impl TestPlatform {
         #[cfg(target_os = "macos")]
         let text_system = Arc::new(crate::platform::mac::MacTextSystem::new());
 
-        #[cfg(target_os = "linux")]
+        #[cfg(any(target_os = "linux", target_os = "freebsd"))]
         let text_system = Arc::new(crate::platform::linux::CosmicTextSystem::new());
 
         #[cfg(target_os = "windows")]
@@ -76,7 +76,7 @@ impl TestPlatform {
             active_display: Rc::new(TestDisplay::new()),
             active_window: Default::default(),
             current_clipboard_item: Mutex::new(None),
-            #[cfg(target_os = "linux")]
+            #[cfg(any(target_os = "linux", target_os = "freebsd"))]
             current_primary_item: Mutex::new(None),
             weak: weak.clone(),
             opened_url: Default::default(),
@@ -291,7 +291,7 @@ impl Platform for TestPlatform {
         false
     }
 
-    #[cfg(target_os = "linux")]
+    #[cfg(any(target_os = "linux", target_os = "freebsd"))]
     fn write_to_primary(&self, item: ClipboardItem) {
         *self.current_primary_item.lock() = Some(item);
     }
@@ -300,7 +300,7 @@ impl Platform for TestPlatform {
         *self.current_clipboard_item.lock() = Some(item);
     }
 
-    #[cfg(target_os = "linux")]
+    #[cfg(any(target_os = "linux", target_os = "freebsd"))]
     fn read_from_primary(&self) -> Option<ClipboardItem> {
         self.current_primary_item.lock().clone()
     }