refactor: rename _ex() -> _ext()

"ex" means "used to be, but no longer is".
"ext" means "extended", which is the intended meaning.
"Ext" is more common not only in the Rust ecosystem but in general.
We already had several people (myself included) asking
what "ex" is supposed to mean.

To reproduce this commit, search and replace `_ex(?!\w)` -> `_ext`.
But don't change `list_transports_ex` because it's public API.

src/message/message_tests.rs

@@ -350,16 +350,16 @@ async fn test_msg_seen_on_imap_when_downloaded() -> Result<()> {
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_pre_and_post_msgs_deleted() -> Result<()> {
     let reorder = false;
-    test_pre_and_post_msgs_deleted_ex(reorder).await
+    test_pre_and_post_msgs_deleted_ext(reorder).await
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_reordered_pre_and_post_msgs_deleted() -> Result<()> {
     let reorder = true;
-    test_pre_and_post_msgs_deleted_ex(reorder).await
+    test_pre_and_post_msgs_deleted_ext(reorder).await
 }
 
-async fn test_pre_and_post_msgs_deleted_ex(reorder: bool) -> Result<()> {
+async fn test_pre_and_post_msgs_deleted_ext(reorder: bool) -> Result<()> {
     let mut tcm = TestContextManager::new();
     let alice = &tcm.alice().await;
     let bob = &tcm.bob().await;