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/indexed_docs/Cargo.toml

@@ -30,6 +30,7 @@ paths.workspace = true
 serde.workspace = true
 strum.workspace = true
 util.workspace = true
+extension_host.workspace = true
 
 [dev-dependencies]
 indoc.workspace = true