Overhaul extension registration (#21083)

This PR overhauls extension registration in order to make it more
modular.

The `extension` crate now contains an `ExtensionHostProxy` that can be
used to register various proxies that the extension host can use to
interact with the rest of the system.

There are now a number of different proxy traits representing the
various pieces of functionality that can be provided by an extension.
The respective crates that provide this functionality can implement
their corresponding proxy trait in order to register a proxy that the
extension host will use to register the bits of functionality provided
by the extension.

Release Notes:

- N/A

crates/zed/Cargo.toml

@@ -19,7 +19,6 @@ activity_indicator.workspace = true
 anyhow.workspace = true
 assets.workspace = true
 assistant.workspace = true
-assistant_slash_command.workspace = true
 async-watch.workspace = true
 audio.workspace = true
 auto_update.workspace = true
@@ -36,12 +35,12 @@ collab_ui.workspace = true
 collections.workspace = true
 command_palette.workspace = true
 command_palette_hooks.workspace = true
-context_servers.workspace = true
 copilot.workspace = true
 db.workspace = true
 diagnostics.workspace = true
 editor.workspace = true
 env_logger.workspace = true
+extension.workspace = true
 extension_host.workspace = true
 extensions_ui.workspace = true
 feature_flags.workspace = true
@@ -56,11 +55,11 @@ go_to_line.workspace = true
 gpui = { workspace = true, features = ["wayland", "x11", "font-kit"] }
 http_client.workspace = true
 image_viewer.workspace = true
-indexed_docs.workspace = true
 inline_completion_button.workspace = true
 install_cli.workspace = true
 journal.workspace = true
 language.workspace = true
+language_extension.workspace = true
 language_model.workspace = true
 language_models.workspace = true
 language_selector.workspace = true
@@ -109,6 +108,7 @@ tasks_ui.workspace = true
 telemetry_events.workspace = true
 terminal_view.workspace = true
 theme.workspace = true
+theme_extension.workspace = true
 theme_selector.workspace = true
 time.workspace = true
 toolchain_selector.workspace = true