Rework context insertion UX (#12360)

- Confirming a completion now runs the command immediately
- Hitting `enter` on a line with a command now runs it
- The output of commands gets folded away and replaced with a custom
placeholder
- Eliminated ambient context

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/b1927a45-52d6-4634-acc9-2ee539c1d89a">

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/editor/src/editor.rs

@@ -1628,6 +1628,7 @@ impl Editor {
                     })
                     .into_any()
             }),
+            merge_adjacent: true,
         };
         let display_map = cx.new_model(|cx| {
             let file_header_size = if show_excerpt_controls { 3 } else { 2 };
@@ -3905,6 +3906,7 @@ impl Editor {
 
         let snippet;
         let text;
+
         if completion.is_snippet() {
             snippet = Some(Snippet::parse(&completion.new_text).log_err()?);
             text = snippet.as_ref().unwrap().text.clone();
@@ -3998,6 +4000,10 @@ impl Editor {
             this.refresh_inline_completion(true, cx);
         });
 
+        if let Some(confirm) = completion.confirm.as_ref() {
+            (confirm)(cx);
+        }
+
         let provider = self.completion_provider.as_ref()?;
         let apply_edits = provider.apply_additional_edits_for_completion(
             buffer_handle,