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/indexed_docs/src/extension_indexed_docs_provider.rs

@@ -29,6 +29,11 @@ impl ExtensionIndexedDocsProviderProxy for IndexedDocsRegistryProxy {
                 ProviderId(provider_id),
             )));
     }
+
+    fn unregister_indexed_docs_provider(&self, provider_id: Arc<str>) {
+        self.indexed_docs_registry
+            .unregister_provider(&ProviderId(provider_id));
+    }
 }
 
 pub struct ExtensionIndexedDocsProvider {