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/registry.rs

@@ -100,6 +100,11 @@ impl ThemeRegistry {
             .ok_or_else(|| anyhow!("theme not found: {}", name))
             .cloned()
     }
+
+    pub fn load_user_themes(&mut self) {
+        #[cfg(not(feature = "importing-themes"))]
+        self.insert_user_theme_familes(crate::all_user_themes());
+    }
 }
 
 impl Default for ThemeRegistry {
@@ -110,9 +115,6 @@ impl Default for ThemeRegistry {
 
         this.insert_theme_families([one_family()]);
 
-        #[cfg(not(feature = "importing-themes"))]
-        this.insert_user_theme_familes(crate::all_user_themes());
-
         this
     }
 }