Rename extension crate to extension_host (#20081)

This PR renames the `extension` crate to `extension_host`.

This is to free up the name so that we can create a smaller-scoped
`extension` crate.

Release Notes:

- N/A

crates/activity_indicator/Cargo.toml

@@ -16,7 +16,7 @@ doctest = false
 anyhow.workspace = true
 auto_update.workspace = true
 editor.workspace = true
-extension.workspace = true
+extension_host.workspace = true
 futures.workspace = true
 gpui.workspace = true
 language.workspace = true