Spina plugins using non-compiled styles don't work properly in admin

[simmerz]

Because the Tailwind stylesheet is pre-compiled in Spina, third party plugins and custom admin code at the app level can't use styles that aren't already compiled into the admin stylesheet.

With the work done on supporting Rails Engines in tailwindcss-rails, this might be an opportune time to look at how to build that support in.

My initial thoughts are to have a default engine stylesheet which is included by a main stylesheet by way of an @import declaration. That app/assets/tailwind/spina.css could then be overridden at the app level to import other stylesheets. For sites that don't need plugins, no override would be needed, but the installer could easily enough add the spina/spina.css file to the main app, which each plugin could then append its import to if needed as part of its own install task.

Presently this is me running my thoughts out to validate them, but this would allow a much more coherent setup with regards to plugin development or extending Spina at the site/app level.

This change would necessitate an upgrade to v4.