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/theme2/src/settings.rs

@@ -117,7 +117,7 @@ impl settings::Settings for ThemeSettings {
         user_values: &[&Self::FileContent],
         cx: &mut AppContext,
     ) -> Result<Self> {
-        let themes = cx.default_global::<Arc<ThemeRegistry>>();
+        let themes = cx.default_global::<ThemeRegistry>();
 
         let mut this = Self {
             ui_font_size: defaults.ui_font_size.unwrap_or(16.).into(),