Generic collections & dayjs for date parsing

[bensmithett]

While other meta.collection values are possible, the posts collection has a bit of special behaviour baked around it.

• It's hardcoded as the collection that generates the JSON Feed
• It's a special prop passed to every page
• It's expected that you set a meta.date whenever you set collection: 'posts'

Change

• All collections are passed to every page in a collections prop
• Every collection is sorted based on the page's meta.date if it exists (parsed & compared with dayjs)
• The JSON Feed is now generated from the collection defined in tropical.feedCollection in package.json (defaults to posts)