Add codespell support (config, workflow to detect/not fix) and make it fix some typos (#903)

More about codespell: https://github.com/codespell-project/codespell .

I personally introduced it to dozens if not hundreds of projects already
and so far only positive feedback.

CI workflow has 'permissions' set only to 'read' so also should be safe.

Let me know if just want to take typo fixes in and get rid of the CI

---------

Signed-off-by: Yaroslav O. Halchenko <debian@onerussian.com>

codex-rs/core/src/codex.rs

@@ -832,7 +832,7 @@ async fn run_turn(
         let prev_id = if store {
             state.previous_response_id.clone()
         } else {
-            // When using ZDR, the Reponses API may send previous_response_id
+            // When using ZDR, the Responses API may send previous_response_id
             // back, but trying to use it results in a 400.
             None
         };

codex-rs/core/src/exec.rs

@@ -318,7 +318,7 @@ pub(crate) async fn spawn_child_async(
     cmd.kill_on_drop(true).spawn()
 }
 
-/// Alternative verison of `spawn_child_async()` that returns
+/// Alternative version of `spawn_child_async()` that returns
 /// `std::process::Child` instead of `tokio::process::Child`. This is useful for
 /// spawning a child process in a thread that is not running a Tokio runtime.
 pub fn spawn_child_sync(