Stop extensions' servers and message loops before removing their files (#34208)

Fixes an issue that caused Windows to fail when removing extension's
directories, as Zed had never stop any related processes.

Now:

* Zed shuts down and waits until the end when the language servers are
shut down

* Adds `impl Drop for WasmExtension` where does
`self.tx.close_channel();` to stop a receiver loop that holds the "lock"
on the extension's work dir.
The extension was dropped, but the channel was not closed for some
reason.

* Does more unregistration to ensure `Arc<WasmExtension>` with the `tx`
does not leak further

* Tidies up the related errors which had never reported a problematic
path before

Release Notes:

- N/A

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>
Co-authored-by: Smit <smit@zed.dev>

crates/assistant_slash_command/src/extension_slash_command.rs

@@ -34,6 +34,11 @@ impl ExtensionSlashCommandProxy for SlashCommandRegistryProxy {
         self.slash_command_registry
             .register_command(ExtensionSlashCommand::new(extension, command), false)
     }
+
+    fn unregister_slash_command(&self, command_name: Arc<str>) {
+        self.slash_command_registry
+            .unregister_command_by_name(&command_name)
+    }
 }
 
 /// An adapter that allows an [`LspAdapterDelegate`] to be used as a [`WorktreeDelegate`].