This commit adds initial support for fresh "plugins". These are bundles
of functionality that can be added to a fresh plugin through a simple
addition in the `main.ts` file.
Right now plugins are only able to hook into the render steps. This
means that they can inject styling and scripts. They can not yet hook
into routing.
Things that I think should be possible now:
- styling integrations for twind, unocss, and probably a few other tools
Things that are not possible yet:
- google analytics middleware
- database connectors
- CMS connectors
The routing additions to plugins can be added in a follow-up though.
