Fix toasts on Windows under WSL 2 (#7137)

Before this: no notifications or toasts when using Codex CLI in WSL 2.

After this: I get toasts from Codex

codex-rs/core/src/env.rs

@@ -0,0 +1,19 @@
+//! Functions for environment detection that need to be shared across crates.
+
+/// Returns true if the current process is running under Windows Subsystem for Linux.
+pub fn is_wsl() -> bool {
+    #[cfg(target_os = "linux")]
+    {
+        if std::env::var_os("WSL_DISTRO_NAME").is_some() {
+            return true;
+        }
+        match std::fs::read_to_string("/proc/version") {
+            Ok(version) => version.to_lowercase().contains("microsoft"),
+            Err(_) => false,
+        }
+    }
+    #[cfg(not(target_os = "linux"))]
+    {
+        false
+    }
+}