feat(shell): add exit_status to get the child's exit status.

plugins/shell/src/process/mod.rs

@@ -80,6 +80,13 @@ impl CommandChild {
         Ok(())
     }
 
+    /// Return the child's exit status if it has already exited. If the child is
+    /// still running, return `Ok(None)`.
+    pub fn exit_status(&self) -> crate::Result<Option<ExitStatus>> {
+        let status = self.inner.try_wait()?;
+        Ok(status.map(|s| ExitStatus { code: s.code() }))
+    }
+
     /// Returns the process pid.
     pub fn pid(&self) -> u32 {
         self.inner.id()