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/zed/Cargo.toml

@@ -15,6 +15,7 @@ name = "zed"
 path = "src/main.rs"
 
 [dependencies]
+assistant_slash_command.workspace = true
 activity_indicator.workspace = true
 anyhow.workspace = true
 assets.workspace = true
@@ -53,6 +54,7 @@ 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