Change behaviour of e.g. foo/index.json.js

[Rich-Harris]

I often find myself in a situation where I want to co-locate the code for routes like these:

• /posts/read-this-now
• /posts/read-this-now.json

The first one can be created by routes/posts/[slug].html or routes/posts/[slug]/index.html. (In all likelihood, it works by fetching /posts/read-this-now.json in preload.)

But the second can only be routes/posts/[slug].json.js, not routes/posts/[slug]/index.json.js, which would be more convenient as it would mean the page and the server route could occupy the same directory.

This would be a minor breaking change but I think it would be more consistent and natural.