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/app.rs

@@ -614,7 +614,7 @@ impl AppContext {
 
     /// Writes data to the primary selection buffer.
     /// Only available on Linux.
-    #[cfg(target_os = "linux")]
+    #[cfg(any(target_os = "linux", target_os = "freebsd"))]
     pub fn write_to_primary(&self, item: ClipboardItem) {
         self.platform.write_to_primary(item)
     }
@@ -626,7 +626,7 @@ impl AppContext {
 
     /// Reads data from the primary selection buffer.
     /// Only available on Linux.
-    #[cfg(target_os = "linux")]
+    #[cfg(any(target_os = "linux", target_os = "freebsd"))]
     pub fn read_from_primary(&self) -> Option<ClipboardItem> {
         self.platform.read_from_primary()
     }