feat: allow plugins to define routes

[jeremydw]

This feature allows plugins to define their own routes that go through the Vite server.

Previously, if you wanted to write a plugin that creates routes and share that plugin across sites within a monorepo, you needed to manually add routes in the routes/... dir that load and provide the routes.

Now, a plugin can write its own routes to construct routes that go through the Vite server. So, plugins can create pages that use import.meta.glob, provide API routes, etc. without copying the routes into the project dir.

The idea is that plugins can be authored for common utilities like design system galleries, custom localization integrations, a custom issue tracker app, etc. -- all using the Vite server without copying the routes files.

• Add integration tests for dev/prod mode.
• Add tests to cover API routes too.