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/editor/src/editor.rs

@@ -2564,7 +2564,7 @@ impl Editor {
         cx.invalidate_character_coordinates();
 
         // Copy selections to primary selection buffer
-        #[cfg(target_os = "linux")]
+        #[cfg(any(target_os = "linux", target_os = "freebsd"))]
         if local {
             let selections = self.selections.all::<usize>(cx);
             let buffer_handle = self.buffer.read(cx).read(cx);

crates/editor/src/element.rs

@@ -650,12 +650,14 @@ impl EditorElement {
             cx.stop_propagation();
         } else if end_selection && pending_nonempty_selections {
             cx.stop_propagation();
-        } else if cfg!(target_os = "linux") && event.button == MouseButton::Middle {
+        } else if cfg!(any(target_os = "linux", target_os = "freebsd"))
+            && event.button == MouseButton::Middle
+        {
             if !text_hitbox.is_hovered(cx) || editor.read_only(cx) {
                 return;
             }
 
-            #[cfg(target_os = "linux")]
+            #[cfg(any(target_os = "linux", target_os = "freebsd"))]
             if EditorSettings::get_global(cx).middle_click_paste {
                 if let Some(text) = cx.read_from_primary().and_then(|item| item.text()) {
                     let point_for_position =