Manage Element Libraries in the Studio Settings

Let's allow to visually manage Elements in the Studio settings.

• Should list existing Element Libraries (Remotion Elements cannot currently be removed because it doesn't require a config file entry)
• Should allow adding an Element Library by URL.
• Should allow removing an Element Library (try following UX conventions established by Skills and Models, we will also migrate Packages to it later)
• Should allow browsing Element Libraries, this should open an iframe for https://www.remotion.dev/elements/libraries, similar to how we allow browsing Elements in an iframe already
• All changes should save to remotion.config.ts
• This should be possible also in browser studio, but not in read-only studio or detached studio /studio-mode
• The icon in settings can be the Elements icon
• The tab can be placed between Skills and Models
• In the dropdown for "Browse Elements...", there can be another option "Manage Elements..." which opens settings (technically it is for managing Element Libraries, not Elements, but I think it's fine). There should be a divider between.