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/extensions_ui/src/extensions_ui.rs

@@ -1,7 +1,15 @@
 mod components;
+mod extension_indexed_docs_provider;
+mod extension_registration_hooks;
+mod extension_slash_command;
 mod extension_suggest;
 mod extension_version_selector;
 
+#[cfg(test)]
+mod extension_store_test;
+
+pub use extension_registration_hooks::ConcreteExtensionRegistrationHooks;
+
 use std::ops::DerefMut;
 use std::sync::OnceLock;
 use std::time::Duration;