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/extension_api/wit/since_v0.0.7/slash-command.wit

@@ -5,6 +5,8 @@ interface slash-command {
         name: string,
         /// The description of the slash command.
         description: string,
+        /// The tooltip text to display for the run button.
+        tooltip-text: string,
         /// Whether this slash command requires an argument.
         requires-argument: bool,
     }