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/editor2/src/editor_tests.rs

@@ -8277,7 +8277,7 @@ pub(crate) fn init_test(cx: &mut TestAppContext, f: fn(&mut AllLanguageSettingsC
     cx.update(|cx| {
         let store = SettingsStore::test(cx);
         cx.set_global(store);
-        theme::init(cx);
+        theme::init(theme::LoadThemes::JustBase, cx);
         client::init_settings(cx);
         language::init(cx);
         Project::init_settings(cx);