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.

deltachat-repl/src/cmdline.rs

@@ -616,7 +616,7 @@ pub async fn cmdline(context: Context, line: &str, chat_id: &mut ChatId) -> Resu
             let sel_chat = sel_chat.as_ref().unwrap();
 
             let time_start = std::time::SystemTime::now();
-            let msglist = chat::get_chat_msgs_ex(
+            let msglist = chat::get_chat_msgs_ext(
                 &context,
                 sel_chat.get_id(),
                 chat::MessageListOptions {