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/paths/src/paths.rs

@@ -20,7 +20,7 @@ pub fn config_dir() -> &'static PathBuf {
                 .join("Zed");
         }
 
-        if cfg!(target_os = "linux") {
+        if cfg!(any(target_os = "linux", target_os = "freebsd")) {
             return if let Ok(flatpak_xdg_config) = std::env::var("FLATPAK_XDG_CONFIG_HOME") {
                 flatpak_xdg_config.into()
             } else {
@@ -41,7 +41,7 @@ pub fn support_dir() -> &'static PathBuf {
             return home_dir().join("Library/Application Support/Zed");
         }
 
-        if cfg!(target_os = "linux") {
+        if cfg!(any(target_os = "linux", target_os = "freebsd")) {
             return if let Ok(flatpak_xdg_data) = std::env::var("FLATPAK_XDG_DATA_HOME") {
                 flatpak_xdg_data.into()
             } else {
@@ -76,7 +76,7 @@ pub fn temp_dir() -> &'static PathBuf {
                 .join("Zed");
         }
 
-        if cfg!(target_os = "linux") {
+        if cfg!(any(target_os = "linux", target_os = "freebsd")) {
             return if let Ok(flatpak_xdg_cache) = std::env::var("FLATPAK_XDG_CACHE_HOME") {
                 flatpak_xdg_cache.into()
             } else {