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/scene.rs

@@ -41,7 +41,10 @@ impl Scene {
     }
 
     #[cfg_attr(
-        all(target_os = "linux", not(any(feature = "x11", feature = "wayland"))),
+        all(
+            any(target_os = "linux", target_os = "freebsd"),
+            not(any(feature = "x11", feature = "wayland"))
+        ),
         allow(dead_code)
     )]
     pub fn paths(&self) -> &[Path<ScaledPixels>] {
@@ -135,7 +138,10 @@ impl Scene {
     }
 
     #[cfg_attr(
-        all(target_os = "linux", not(any(feature = "x11", feature = "wayland"))),
+        all(
+            any(target_os = "linux", target_os = "freebsd"),
+            not(any(feature = "x11", feature = "wayland"))
+        ),
         allow(dead_code)
     )]
     pub(crate) fn batches(&self) -> impl Iterator<Item = PrimitiveBatch> {
@@ -167,7 +173,10 @@ impl Scene {
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd, Default)]
 #[cfg_attr(
-    all(target_os = "linux", not(any(feature = "x11", feature = "wayland"))),
+    all(
+        any(target_os = "linux", target_os = "freebsd"),
+        not(any(feature = "x11", feature = "wayland"))
+    ),
     allow(dead_code)
 )]
 pub(crate) enum PrimitiveKind {
@@ -225,7 +234,10 @@ impl Primitive {
 }
 
 #[cfg_attr(
-    all(target_os = "linux", not(any(feature = "x11", feature = "wayland"))),
+    all(
+        any(target_os = "linux", target_os = "freebsd"),
+        not(any(feature = "x11", feature = "wayland"))
+    ),
     allow(dead_code)
 )]
 struct BatchIterator<'a> {
@@ -415,7 +427,10 @@ impl<'a> Iterator for BatchIterator<'a> {
 
 #[derive(Debug)]
 #[cfg_attr(
-    all(target_os = "linux", not(any(feature = "x11", feature = "wayland"))),
+    all(
+        any(target_os = "linux", target_os = "freebsd"),
+        not(any(feature = "x11", feature = "wayland"))
+    ),
     allow(dead_code)
 )]
 pub(crate) enum PrimitiveBatch<'a> {