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/collab2/src/tests/test_server.rs

@@ -224,7 +224,7 @@ impl TestServer {
         });
 
         cx.update(|cx| {
-            theme::init(cx);
+            theme::init(theme::LoadThemes::JustBase, cx);
             Project::init(&client, cx);
             client::init(&client, cx);
             language::init(cx);