zed: Allocate more rayon threads depending on available parallelism (#43235)

While zed itself is not a heavy user of rayon, wasmtime is, especially
for compilation. This change is similar to the rayon default but we
halve the number of threads still so we don't spawn too many threads
overall.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/remote_server/src/unix.rs

@@ -373,7 +373,7 @@ pub fn execute_run(
     let listeners = ServerListeners::new(stdin_socket, stdout_socket, stderr_socket)?;
 
     rayon::ThreadPoolBuilder::new()
-        .num_threads(4)
+        .num_threads(std::thread::available_parallelism().map_or(1, |n| n.get().div_ceil(2)))
         .stack_size(10 * 1024 * 1024)
         .thread_name(|ix| format!("RayonWorker{}", ix))
         .build_global()

crates/zed/src/main.rs

@@ -265,7 +265,7 @@ pub fn main() {
     }
 
     rayon::ThreadPoolBuilder::new()
-        .num_threads(4)
+        .num_threads(std::thread::available_parallelism().map_or(1, |n| n.get().div_ceil(2)))
         .stack_size(10 * 1024 * 1024)
         .thread_name(|ix| format!("RayonWorker{}", ix))
         .build_global()