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
