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/src/extension_store.rs

@@ -1183,6 +1183,7 @@ impl ExtensionStore {
                                 command: crate::wit::SlashCommand {
                                     name: slash_command_name.to_string(),
                                     description: slash_command.description.to_string(),
+                                    tooltip_text: slash_command.tooltip_text.to_string(),
                                     requires_argument: slash_command.requires_argument,
                                 },
                                 extension: wasm_extension.clone(),