Parameterize theme2::init to allow loading just the base theme (#3345)

This PR adds a parameter to the `theme2::init` method to indicate what
the theme-loading behavior should be.

This allows us to indicate when we want to load all of the additional
built-in user themes (like in the Zed binary and in the storybook), and
when we don't want to load the user themes (like in tests).

We're using an enum over just a `bool` here for clarity at the call
site.

Release Notes:

- N/A

crates/workspace2/src/workspace2.rs

@@ -355,7 +355,7 @@ impl AppState {
         let user_store = cx.build_model(|cx| UserStore::new(client.clone(), http_client, cx));
         let workspace_store = cx.build_model(|cx| WorkspaceStore::new(client.clone(), cx));
 
-        theme2::init(cx);
+        theme2::init(theme2::LoadThemes::JustBase, cx);
         client2::init(&client, cx);
         crate::init_settings(cx);