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/cli/src/main.rs

@@ -1,4 +1,7 @@
-#![cfg_attr(any(target_os = "linux", target_os = "windows"), allow(dead_code))]
+#![cfg_attr(
+    any(target_os = "linux", target_os = "freebsd", target_os = "windows"),
+    allow(dead_code)
+)]
 
 use anyhow::{Context, Result};
 use clap::Parser;
@@ -88,7 +91,7 @@ fn parse_path_with_position(argument_str: &str) -> anyhow::Result<String> {
 
 fn main() -> Result<()> {
     // Exit flatpak sandbox if needed
-    #[cfg(target_os = "linux")]
+    #[cfg(any(target_os = "linux", target_os = "freebsd"))]
     {
         flatpak::try_restart_to_host();
         flatpak::ld_extra_libs();
@@ -106,7 +109,7 @@ fn main() -> Result<()> {
     }
     let args = Args::parse();
 
-    #[cfg(target_os = "linux")]
+    #[cfg(any(target_os = "linux", target_os = "freebsd"))]
     let args = flatpak::set_bin_if_no_escape(args);
 
     let app = Detect::detect(args.zed.as_deref()).context("Bundle detection")?;
@@ -220,7 +223,7 @@ fn main() -> Result<()> {
     Ok(())
 }
 
-#[cfg(target_os = "linux")]
+#[cfg(any(target_os = "linux", target_os = "freebsd"))]
 mod linux {
     use std::{
         env,
@@ -344,7 +347,7 @@ mod linux {
     }
 }
 
-#[cfg(target_os = "linux")]
+#[cfg(any(target_os = "linux", target_os = "freebsd"))]
 mod flatpak {
     use std::ffi::OsString;
     use std::path::PathBuf;