Move ExtensionStore tests back to extension_host (#20682)

This PR moves the tests for the `ExtensionStore` back into the
`extension_host` crate.

We now have a separate `TestExtensionRegistrationHooks` to use in the
test that implements the minimal required functionality needed for the
tests. This means that we can depend on the `theme` crate only in the
tests.

Release Notes:

- N/A

crates/extension_host/src/extension_host.rs

@@ -2,6 +2,9 @@ pub mod extension_lsp_adapter;
 pub mod extension_settings;
 pub mod wasm_host;
 
+#[cfg(test)]
+mod extension_store_test;
+
 use crate::{extension_lsp_adapter::ExtensionLspAdapter, wasm_host::wit};
 use anyhow::{anyhow, bail, Context as _, Result};
 use async_compression::futures::bufread::GzipDecoder;