Merge Swell-generated and user-supplied routes

[mewa]

Currently there's no way for the end-user to supply their own routes to be generated, as is the case when implementing e.g. a blog section.

This PR updates the Swell plugin so that it merges the Swell-generated routes with the routes already supplied by the user in nuxt.config.js.

[netlify[bot]]

👷 Deploy request for swell-origin-theme pending review.

Visit the deploys page to approve it

Name	Link
🔨 Latest commit	89b7150ecf3a81f350f878134161dcd2ef3e1edb