Implement radix-themes as a plugin instead of tightly integrated in the core

[masenf]

Describe the Enhancement you want

For apps that do not use radix-themes, there is no way to opt-out of the radix themes js/css bundle that increases overall app size. Radix themes should only be included if the app is making use of radix themes components.

What needs to happen

• Create a Plugin that encapsulates any radix-themes functionality that is currently in App (for example, expose the theme= kwarg on the plugin's constructor)
• The RadixThemesPlugin should not be enabled by default
• If theme= is supplied to App, the plugin should be enabled, with a deprecation warning.
• If any radix themes components are used in any page, the plugin should be enabled, with a deprecation warning.

[linear]

ENG-9339