Extension refactor (#20305)

This contains the main changes to the extensions crate from #20049. The
primary goal here is removing dependencies that we can't include on the
remote.


Release Notes:

- N/A

---------

Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Marshall <marshall@zed.dev>

crates/remote_server/src/remote_editing_tests.rs

@@ -1187,6 +1187,9 @@ pub async fn init_test(
     cx.update(|cx| {
         release_channel::init(SemanticVersion::default(), cx);
     });
+    server_cx.update(|cx| {
+        release_channel::init(SemanticVersion::default(), cx);
+    });
     init_logger();
 
     let (opts, ssh_server_client) = SshRemoteClient::fake_server(cx, server_cx);