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/display_map/inlay_map.rs

@@ -1891,6 +1891,6 @@ mod tests {
     fn init_test(cx: &mut AppContext) {
         let store = SettingsStore::test(cx);
         cx.set_global(store);
-        theme::init(cx);
+        theme::init(theme::LoadThemes::JustBase, cx);
     }
 }