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/extensions_ui/Cargo.toml

@@ -20,7 +20,7 @@ client.workspace = true
 collections.workspace = true
 db.workspace = true
 editor.workspace = true
-extension.workspace = true
+extension_host.workspace = true
 fs.workspace = true
 fuzzy.workspace = true
 gpui.workspace = true