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/command_palette2/src/command_palette.rs

@@ -456,7 +456,7 @@ mod tests {
     fn init_test(cx: &mut TestAppContext) -> Arc<AppState> {
         cx.update(|cx| {
             let app_state = AppState::test(cx);
-            theme::init(cx);
+            theme::init(theme::LoadThemes::JustBase, cx);
             language::init(cx);
             editor::init(cx);
             workspace::init(app_state.clone(), cx);