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/zed2/src/main.rs

@@ -140,7 +140,7 @@ fn main() {
 
         cx.set_global(client.clone());
 
-        theme::init(cx);
+        theme::init(theme::LoadThemes::All, cx);
         project::Project::init(&client, cx);
         client::init(&client, cx);
         command_palette::init(cx);