feat: allow plugins to define routes

[jeremydw]

This feature allows plugins to define their own routes that go through the Vite server. Previously, the two ways a plugin could define routes were:

1. For the site to manually import the plugin in the routes/... dir. (Compatible with the Vite server.)
2. For the plugin to use configureServer and manually construct the routes and responses. (Not compatible with the Vite server.)

• Add test for the SSG build.
• Add a new useRouter that wraps useRequestContext.